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/system/BUILD.gn

@@ -29,8 +29,8 @@ rtc_library("file_wrapper") {
     "..:checks",
     "..:criticalsection",
     "..:safe_conversions",
+    "//third_party/abseil-cpp/absl/strings:string_view",
   ]
-  absl_deps = [ "//third_party/abseil-cpp/absl/strings" ]
 }
 
 if (rtc_include_tests) {

webrtc/rtc_base/system/file_wrapper.cc

@@ -14,6 +14,7 @@
 
 #include <cerrno>
 #include <cstdint>
+#include <optional>
 #include <string>
 
 #include "absl/strings/string_view.h"
@@ -81,20 +82,20 @@ bool FileWrapper::SeekTo(int64_t position) {
   return fseek(file_, rtc::checked_cast<long>(position), SEEK_SET) == 0;
 }
 
-long FileWrapper::FileSize() {
+std::optional<size_t> FileWrapper::FileSize() {
   if (file_ == nullptr)
-    return -1;
+    return std::nullopt;
   long original_position = ftell(file_);
   if (original_position < 0)
-    return -1;
+    return std::nullopt;
   int seek_error = fseek(file_, 0, SEEK_END);
   if (seek_error)
-    return -1;
+    return std::nullopt;
   long file_size = ftell(file_);
   seek_error = fseek(file_, original_position, SEEK_SET);
   if (seek_error)
-    return -1;
-  return file_size;
+    return std::nullopt;
+  return rtc::checked_cast<size_t>(file_size);
 }
 
 bool FileWrapper::Flush() {

webrtc/rtc_base/system/file_wrapper.h

@@ -14,8 +14,8 @@
 #include <stddef.h>
 #include <stdint.h>
 #include <stdio.h>
-#include <stdint.h>
 
+#include <optional>
 #include <string>
 
 #include "absl/strings/string_view.h"
@@ -90,7 +90,7 @@ class FileWrapper final {
   // Returns the file size or -1 if a size could not be determined.
   // (A file size might not exists for non-seekable files or file-like
   // objects, for example /dev/tty on unix.)
-  long FileSize();
+  std::optional<size_t> FileSize();
 
   // Returns number of bytes read. Short count indicates EOF or error.
   size_t Read(void* buf, size_t length);