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/api/audio_codecs/audio_decoder.h

@@ -15,9 +15,9 @@
 #include <stdint.h>
 
 #include <memory>
+#include <optional>
 #include <vector>
 
-#include "absl/types/optional.h"
 #include "api/array_view.h"
 #include "rtc_base/buffer.h"
 
@@ -57,11 +57,11 @@ class AudioDecoder {
 
     // Decodes this frame of audio and writes the result in `decoded`.
     // `decoded` must be large enough to store as many samples as indicated by a
-    // call to Duration() . On success, returns an absl::optional containing the
+    // call to Duration() . On success, returns an std::optional containing the
     // total number of samples across all channels, as well as whether the
     // decoder produced comfort noise or speech. On failure, returns an empty
-    // absl::optional. Decode may be called at most once per frame object.
-    virtual absl::optional<DecodeResult> Decode(
+    // std::optional. Decode may be called at most once per frame object.
+    virtual std::optional<DecodeResult> Decode(
         rtc::ArrayView<int16_t> decoded) const = 0;
   };