[Chromecast] Replace Unretained(this) with WeakPtr in MediaPipelineBackendForMixer.

BUG=internal b/117194691

Test=Cast from Youtube to Google Home Hub.(Since we could not reproduce
the bug in the office, this is only a general cast test.)

Change-Id: I2f9ac0079bc3d5846dcd5f67a860796b7340fb6d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1529998
Commit-Queue: Ziyang Cheng <ziyangch@chromium.org>
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642660}

chromecast/media/cma/backend/media_pipeline_backend_for_mixer.cc

@@ -37,9 +37,13 @@ namespace media {
 
 MediaPipelineBackendForMixer::MediaPipelineBackendForMixer(
     const MediaPipelineDeviceParams& params)
-    : state_(kStateUninitialized), params_(params) {}
+    : state_(kStateUninitialized), params_(params), weak_factory_(this) {
+  weak_this_ = weak_factory_.GetWeakPtr();
+}
 
-MediaPipelineBackendForMixer::~MediaPipelineBackendForMixer() {}
+MediaPipelineBackendForMixer::~MediaPipelineBackendForMixer() {
+  DCHECK(GetTaskRunner()->RunsTasksInCurrentSequence());
+}
 
 MediaPipelineBackendForMixer::AudioDecoder*
 MediaPipelineBackendForMixer::CreateAudioDecoder() {
@@ -262,7 +266,7 @@ void MediaPipelineBackendForMixer::VideoReadyToPlay() {
   GetTaskRunner()->PostTask(
       FROM_HERE,
       base::BindOnce(&MediaPipelineBackendForMixer::OnVideoReadyToPlay,
-                     base::Unretained(this)));
+                     weak_this_));
 }
 
 void MediaPipelineBackendForMixer::OnVideoReadyToPlay() {

chromecast/media/cma/backend/media_pipeline_backend_for_mixer.h

@@ -10,6 +10,7 @@
 
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
+#include "base/memory/weak_ptr.h"
 #include "chromecast/media/cma/backend/video_decoder_for_mixer.h"
 #include "chromecast/public/media/media_pipeline_backend.h"
 #include "chromecast/public/media/media_pipeline_device_params.h"
@@ -99,6 +100,9 @@ class MediaPipelineBackendForMixer : public MediaPipelineBackend,
   bool playback_started_ = false;
   float starting_playback_rate_ = 1.0;
 
+  base::WeakPtr<MediaPipelineBackendForMixer> weak_this_;
+  base::WeakPtrFactory<MediaPipelineBackendForMixer> weak_factory_;
+
   DISALLOW_COPY_AND_ASSIGN(MediaPipelineBackendForMixer);
 };