Add MutexTryLocker in AudioBufferSourceHandler::PropagateSilence()

The access to |shared_buffer_| in AudioBufferSourceHandler needs to be
controlled by |process_lock_|. This CL adds one into PropagateSilence()
method.


Bug: 1167472
Test: ToT crashes with the repro case, but it doesn't with this patch.
Change-Id: I52f95d52889c9b809507c88a16c61f0e85b36f3b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2638097
Commit-Queue: Hongchan Choi <hongchan@chromium.org>
Reviewed-by: Raymond Toy <rtoy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845334}

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

@@ -653,7 +653,21 @@ double AudioBufferSourceHandler::GetMinPlaybackRate() {
 }
 
 bool AudioBufferSourceHandler::PropagatesSilence() const {
-  return !IsPlayingOrScheduled() || HasFinished() || !shared_buffer_.get();
+  DCHECK(Context()->IsAudioThread());
+
+  if (!IsPlayingOrScheduled() || HasFinished())
+    return true;
+
+  // Protect |shared_buffer_| with tryLock because it can be accessed by the
+  // main thread.
+  MutexTryLocker try_locker(process_lock_);
+  if (try_locker.Locked()) {
+    return !shared_buffer_.get();
+  } else {
+    // Can't get lock. Assume |shared_buffer_| exists, so return false to
+    // indicate this node is (or might be) outputting non-zero samples.
+    return false;
+  }
 }
 
 void AudioBufferSourceHandler::HandleStoppableSourceNode() {