Invoke VideoRenderer flush callback via PostTask

This is necessary to ensure flush callback is called outside of
VideoRenderer::lock_ thus allowing the callback code to access
VideoRender methods. AudioRendererImpl already does the same thing.

Review-Url: https://codereview.chromium.org/2012903002
Cr-Commit-Position: refs/heads/master@{#396963}

media/renderers/video_renderer_impl.cc

@@ -497,7 +497,10 @@ void VideoRendererImpl::AttemptRead_Locked() {
 }
 
 void VideoRendererImpl::OnVideoFrameStreamResetDone() {
-  base::AutoLock auto_lock(lock_);
+  // We don't need to acquire the |lock_| here, because we can only get here
+  // when Flush is in progress, so rendering and video sink must be stopped.
+  DCHECK(task_runner_->BelongsToCurrentThread());
+  DCHECK(!sink_started_);
   DCHECK_EQ(kFlushing, state_);
   DCHECK(!received_end_of_stream_);
   DCHECK(!rendered_end_of_stream_);

media/renderers/video_renderer_impl_unittest.cc

@@ -573,6 +573,27 @@ TEST_F(VideoRendererImplTest, FlushWithNothingBuffered) {
   Destroy();
 }
 
+// Verify that the flush callback is invoked outside of VideoRenderer lock, so
+// we should be able to call other renderer methods from the Flush callback.
+static void VideoRendererImplTest_FlushDoneCB(VideoRendererImplTest* test,
+                                              VideoRenderer* renderer,
+                                              const base::Closure& success_cb) {
+  test->QueueFrames("0 10 20 30");
+  renderer->StartPlayingFrom(base::TimeDelta::FromSeconds(0));
+  success_cb.Run();
+}
+
+TEST_F(VideoRendererImplTest, FlushCallbackNoLock) {
+  Initialize();
+  StartPlayingFrom(0);
+  WaitableMessageLoopEvent event;
+  renderer_->Flush(
+      base::Bind(&VideoRendererImplTest_FlushDoneCB, base::Unretained(this),
+                 base::Unretained(renderer_.get()), event.GetClosure()));
+  event.RunAndWait();
+  Destroy();
+}
+
 TEST_F(VideoRendererImplTest, DecodeError_Playing) {
   Initialize();
   QueueFrames("0 10 20 30");