Use SupportsWeakPtr for messaging from rendering thread to main thread

In cross-thread messaging, the associated execution context can be
already gone when a posted task is performed sometime later in the task
runner's queue.

By using WeakPtr, the task runner will not perform a scheduled task
in the queue when the target object is invalid.

Test: Locally confirmed that the repro does not crash.
Bug: 1057627
Change-Id: I51737594c918f6a4924c9a7ffe30db3e8de9a683
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2082897
Commit-Queue: Hongchan Choi <hongchan@chromium.org>
Reviewed-by: Raymond Toy <rtoy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746613}

third_party/blink/renderer/modules/webaudio/audio_scheduled_source_node.cc

@@ -250,7 +250,7 @@ void AudioScheduledSourceHandler::Finish() {
   PostCrossThreadTask(
       *task_runner_, FROM_HERE,
       CrossThreadBindOnce(&AudioScheduledSourceHandler::NotifyEnded,
-                          WrapRefCounted(this)));
+                          AsWeakPtr()));
 }
 
 void AudioScheduledSourceHandler::NotifyEnded() {

third_party/blink/renderer/modules/webaudio/audio_scheduled_source_node.h

@@ -30,6 +30,7 @@
 #define THIRD_PARTY_BLINK_RENDERER_MODULES_WEBAUDIO_AUDIO_SCHEDULED_SOURCE_NODE_H_
 
 #include <atomic>
+#include "base/memory/weak_ptr.h"
 #include "third_party/blink/renderer/bindings/core/v8/active_script_wrappable.h"
 #include "third_party/blink/renderer/modules/webaudio/audio_node.h"
 
@@ -38,7 +39,9 @@ namespace blink {
 class BaseAudioContext;
 class AudioBus;
 
-class AudioScheduledSourceHandler : public AudioHandler {
+class AudioScheduledSourceHandler
+    : public AudioHandler,
+      public base::SupportsWeakPtr<AudioScheduledSourceHandler> {
  public:
   // These are the possible states an AudioScheduledSourceNode can be in:
   //