Reenable SeekWhilePlaying

Currently, MediaRemotingIntegrationTest.SeekWhilePlaying is flaky, and
has been disabled. After looking at the logs, the primary reason is that
media::PipelineIntegrationTestBase::OnEnded is called multiple times,
resulting in a DCHECK and test failure. The stack for the second
OnEnded call is:

media::PipelineImpl::RendererWrapper::CheckPlaybackEnded
media::AudioRendererImpl::Render

Logs:
https://logs.chromium.org/logs/chromium/buildbucket/cr-buildbucket.appspot.com/8890990219267312400/+/steps/media_unittests__with_patch_/0/logs/Flaky_failure:_MediaRemotingIntegrationTest.SeekWhilePlaying__status_CRASH_SUCCESS_/0

Turns out, this is actually a bug in the AudioRendererImpl, where the
old method for determining if the content is ended is flaky, resulting
in multiple calls to OnEnded. This bug has been resolved by a change made on the
AudioRendererImpl, moving to an explicit EndedPlayback signal:

https://chromium-review.googlesource.com/c/chromium/src/+/2021292

Bug: 1043812
Change-Id: I8ca939e5166c3fa3c40f33c7a4e34f3a29d0e06b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2258342
Commit-Queue: Jordan Bayles <jophba@chromium.org>
Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781482}

media/remoting/integration_test.cc

@@ -72,8 +72,7 @@ TEST_F(MediaRemotingIntegrationTest, MediaSource_ConfigChange_WebM) {
   Stop();
 }
 
-// Flaky: http://crbug.com/1043812.
-TEST_F(MediaRemotingIntegrationTest, DISABLED_SeekWhilePlaying) {
+TEST_F(MediaRemotingIntegrationTest, SeekWhilePlaying) {
   ASSERT_EQ(PIPELINE_OK, Start("bear-320x240.webm"));
 
   base::TimeDelta duration(pipeline_->GetMediaDuration());