Bug 1294615. Part 1 - Assert mQueuedSeek.Exists() is false in InitiateSeek(). r=kaku

InitiateSeek() is called from several places where we can prove mQueuedSeek.Exists() is false:
1. MaybeFinishDecodeFirstFrame(): mQueuedSeek is moved when calling InitiateSeek().
2. Seek() rejects mQueuedSeek before calling InitiateSeek().
3. StartDecoding(): mQueuedSeek is moved when calling InitiateSeek().
4. VisibilityChanged(): it doesn't call InitiateSeek() when mQueuedSeek.Exists() is true.

MozReview-Commit-ID: BriPw0VID5O

--HG--
extra : rebase_source : 1417b108eab1cf9cf332e55e77077d7947505d27

dom/media/MediaDecoderStateMachine.cpp

@@ -1614,6 +1614,7 @@ MediaDecoderStateMachine::InitiateSeek(SeekJob aSeekJob)
            &MediaDecoderStateMachine::OnSeekTaskResolved,
            &MediaDecoderStateMachine::OnSeekTaskRejected));
 
+  MOZ_ASSERT(!mQueuedSeek.Exists());
   MOZ_ASSERT(!mCurrentSeek.Exists());
   mCurrentSeek = Move(aSeekJob);
   return mCurrentSeek.mPromise.Ensure(__func__);