message: Make internal_source argument to FrameEncodeMetadataWriter::OnEncoderInit optional
  Cr-Commit-Position: refs/heads/main@{#35308}
diff: @@ -28,7 +28,7 @@ class FrameEncodeMetadataWriter {
  explicit FrameEncodeMetadataWriter(EncodedImageCallback* frame_drop_callback);
  ~FrameEncodeMetadataWriter();
  - void OnEncoderInit(const VideoCodec& codec, bool internal_source);
  + void OnEncoderInit(const VideoCodec& codec, bool internal_source = false);
  void OnSetRates(const VideoBitrateAllocation& bitrate_allocation,
  uint32_t framerate_fps);
  