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/synchronization/BUILD.gn

@@ -36,15 +36,18 @@ rtc_source_set("mutex") {
     "..:macromagic",
     "..:platform_thread_types",
     "../system:no_unique_address",
+    "//third_party/abseil-cpp/absl/base:core_headers",
   ]
-  absl_deps = [ "//third_party/abseil-cpp/absl/base:core_headers" ]
   if (rtc_use_absl_mutex) {
-    absl_deps += [ "//third_party/abseil-cpp/absl/synchronization" ]
+    deps += [ "//third_party/abseil-cpp/absl/synchronization" ]
   }
 }
 
 rtc_library("sequence_checker_internal") {
-  visibility = [ "../../api:sequence_checker" ]
+  visibility = [
+    "../../api:rtc_api_unittests",
+    "../../api:sequence_checker",
+  ]
   sources = [
     "sequence_checker_internal.cc",
     "sequence_checker_internal.h",
@@ -65,8 +68,8 @@ rtc_library("yield_policy") {
     "yield_policy.cc",
     "yield_policy.h",
   ]
-  deps = [ "..:checks" ]
-  absl_deps = [
+  deps = [
+    "..:checks",
     "//third_party/abseil-cpp/absl/base:config",
     "//third_party/abseil-cpp/absl/base:core_headers",
   ]

webrtc/rtc_base/synchronization/mutex_pthread.h

@@ -76,8 +76,8 @@ class RTC_LOCKABLE MutexImpl final {
     }
 
    private:
-    // Use two separate primitive types, rather than absl::optional, since the
-    // data race described below might invalidate absl::optional invariants.
+    // Use two separate primitive types, rather than std::optional, since the
+    // data race described below might invalidate std::optional invariants.
     bool is_owned_ = false;
     pthread_t latest_owner_ = pthread_self();
 #endif