Bug 1330889. Part 4 - remove the mState checks from MaybeStartPlayback(). r=kikuo

The same reason as P2.

MozReview-Commit-ID: GqWgiXjlTrb

--HG--
extra : rebase_source : e2664650aaf3d209af1baf539cb0bb860f701fdc

dom/media/MediaDecoderStateMachine.cpp

@@ -2768,8 +2768,6 @@ void MediaDecoderStateMachine::MaybeStartPlayback()
   MOZ_ASSERT(OnTaskQueue());
   // Should try to start playback only after decoding first frames.
   MOZ_ASSERT(mSentFirstFrameLoadedEvent);
-  MOZ_ASSERT(mState == DECODER_STATE_DECODING ||
-             mState == DECODER_STATE_COMPLETED);
 
   if (IsPlaying()) {
     // Logging this case is really spammy - don't do it.