Media Pipeline: Fire seek callback after calling StartPlayingFrom().

The seek callback can trigger "seeked" event in JS. In the current code, seek
callback is fired before calling StartPlayingFrom(), this could cause the
following race condition to happen:
1, Seek callback is fired on the media thread, and posted to the main thread.
2, "seeked" event is fired in JS. currentTime() is called on the main thread by
   the JS player.
3, As a result of (2), AudioRendererImpl::CurrentMediaTime() is called on the
   main thread.
4, Renderer::StartPlayingFrom() is called on the media thread, which calls
   AudioRendererImpl::SetMediaTime().

As you can see, CurrentMediaTime() is called before SetMediaTime(). Hence
step (3) returns an old media time.

This CL makes sure we call StartPlayingFrom() before we fire the seek callback,
so that SetMediaTime() is always called before we fire "seeked" event.

BUG=117921,435342
TEST=Updated PipelineIntegrationTestBase and enabled two tests to cover this.

Review URL: https://codereview.chromium.org/766633003

Cr-Commit-Position: refs/heads/master@{#306259}

1 files changed, 1 insertions, 1 deletions

diff --git a/media/blink/webmediaplayer_impl.cc b/media/blink/webmediaplayer_impl.cc
index e5f73f0..74db5fe 100644
--- a/media/blink/webmediaplayer_impl.cc
+++ b/media/blink/webmediaplayer_impl.cc
@@ -325,7 +325,7 @@ bool WebMediaPlayerImpl::supportsSave() const {
 }
 
 void WebMediaPlayerImpl::seek(double seconds) {
-  DVLOG(1) << __FUNCTION__ << "(" << seconds << ")";
+  DVLOG(1) << __FUNCTION__ << "(" << seconds << "s)";
   DCHECK(main_task_runner_->BelongsToCurrentThread());
 
   ended_ = false;