Bump to WebRTC M131 release

Ongoing fixes and improvements, transient suppressor is gone. Also,
dropping isac because it doesn't seem to be useful, and is just build
system deadweight now.

Upstream references:

  Version: 131.0.6778.200
  WebRTC: 79aff54b0fa9238ce3518dd9eaf9610cd6f22e82
  Chromium: 2a19506ad24af755f2a215a4c61f775393e0db42

webrtc/rtc_base/experiments/field_trial_parser.cc

@@ -60,7 +60,7 @@ void ParseFieldTrial(
   while (!tail.empty()) {
     size_t key_end = tail.find_first_of(",:");
     absl::string_view key = tail.substr(0, key_end);
-    absl::optional<std::string> opt_value;
+    std::optional<std::string> opt_value;
     if (key_end == absl::string_view::npos) {
       tail = "";
     } else if (tail[key_end] == ':') {
@@ -112,17 +112,17 @@ void ParseFieldTrial(
 }
 
 template <>
-absl::optional<bool> ParseTypedParameter<bool>(absl::string_view str) {
+std::optional<bool> ParseTypedParameter<bool>(absl::string_view str) {
   if (str == "true" || str == "1") {
     return true;
   } else if (str == "false" || str == "0") {
     return false;
   }
-  return absl::nullopt;
+  return std::nullopt;
 }
 
 template <>
-absl::optional<double> ParseTypedParameter<double>(absl::string_view str) {
+std::optional<double> ParseTypedParameter<double>(absl::string_view str) {
   double value;
   char unit[2]{0, 0};
   if (sscanf(std::string(str).c_str(), "%lf%1s", &value, unit) >= 1) {
@@ -130,56 +130,56 @@ absl::optional<double> ParseTypedParameter<double>(absl::string_view str) {
       return value / 100;
     return value;
   } else {
-    return absl::nullopt;
+    return std::nullopt;
   }
 }
 
 template <>
-absl::optional<int> ParseTypedParameter<int>(absl::string_view str) {
+std::optional<int> ParseTypedParameter<int>(absl::string_view str) {
   int64_t value;
   if (sscanf(std::string(str).c_str(), "%" SCNd64, &value) == 1) {
     if (rtc::IsValueInRangeForNumericType<int, int64_t>(value)) {
       return static_cast<int>(value);
     }
   }
-  return absl::nullopt;
+  return std::nullopt;
 }
 
 template <>
-absl::optional<unsigned> ParseTypedParameter<unsigned>(absl::string_view str) {
+std::optional<unsigned> ParseTypedParameter<unsigned>(absl::string_view str) {
   int64_t value;
   if (sscanf(std::string(str).c_str(), "%" SCNd64, &value) == 1) {
     if (rtc::IsValueInRangeForNumericType<unsigned, int64_t>(value)) {
       return static_cast<unsigned>(value);
     }
   }
-  return absl::nullopt;
+  return std::nullopt;
 }
 
 template <>
-absl::optional<std::string> ParseTypedParameter<std::string>(
+std::optional<std::string> ParseTypedParameter<std::string>(
     absl::string_view str) {
   return std::string(str);
 }
 
 template <>
-absl::optional<absl::optional<bool>> ParseTypedParameter<absl::optional<bool>>(
+std::optional<std::optional<bool>> ParseTypedParameter<std::optional<bool>>(
     absl::string_view str) {
   return ParseOptionalParameter<bool>(str);
 }
 template <>
-absl::optional<absl::optional<int>> ParseTypedParameter<absl::optional<int>>(
+std::optional<std::optional<int>> ParseTypedParameter<std::optional<int>>(
     absl::string_view str) {
   return ParseOptionalParameter<int>(str);
 }
 template <>
-absl::optional<absl::optional<unsigned>>
-ParseTypedParameter<absl::optional<unsigned>>(absl::string_view str) {
+std::optional<std::optional<unsigned>>
+ParseTypedParameter<std::optional<unsigned>>(absl::string_view str) {
   return ParseOptionalParameter<unsigned>(str);
 }
 template <>
-absl::optional<absl::optional<double>>
-ParseTypedParameter<absl::optional<double>>(absl::string_view str) {
+std::optional<std::optional<double>> ParseTypedParameter<std::optional<double>>(
+    absl::string_view str) {
   return ParseOptionalParameter<double>(str);
 }
 
@@ -197,10 +197,10 @@ webrtc::FieldTrialFlag::operator bool() const {
   return value_;
 }
 
-bool FieldTrialFlag::Parse(absl::optional<std::string> str_value) {
+bool FieldTrialFlag::Parse(std::optional<std::string> str_value) {
   // Only set the flag if there is no argument provided.
   if (str_value) {
-    absl::optional<bool> opt_value = ParseTypedParameter<bool>(*str_value);
+    std::optional<bool> opt_value = ParseTypedParameter<bool>(*str_value);
     if (!opt_value)
       return false;
     value_ = *opt_value;
@@ -224,14 +224,14 @@ AbstractFieldTrialEnum::AbstractFieldTrialEnum(const AbstractFieldTrialEnum&) =
     default;
 AbstractFieldTrialEnum::~AbstractFieldTrialEnum() = default;
 
-bool AbstractFieldTrialEnum::Parse(absl::optional<std::string> str_value) {
+bool AbstractFieldTrialEnum::Parse(std::optional<std::string> str_value) {
   if (str_value) {
     auto it = enum_mapping_.find(*str_value);
     if (it != enum_mapping_.end()) {
       value_ = it->second;
       return true;
     }
-    absl::optional<int> value = ParseTypedParameter<int>(*str_value);
+    std::optional<int> value = ParseTypedParameter<int>(*str_value);
     if (value.has_value() &&
         (valid_values_.find(*value) != valid_values_.end())) {
       value_ = *value;