Bug 1278228 - use MOZ_DIAGNOSTIC_ASSERT() to confirm that AccurateVideoOnly seek might reach MDSM::InitiateSeek(); r=jwwang

MozReview-Commit-ID: LLZn65yZib5

--HG--
extra : transplant_source : %DB%1E%01%0B%0D.%B1%A3%C6%21M%E1e%07S%0E%BCn%13y

dom/media/MediaDecoderStateMachine.cpp

@@ -1618,7 +1618,10 @@ MediaDecoderStateMachine::InitiateSeek(SeekJob aSeekJob)
                                       Move(aSeekJob), mInfo, Duration(),
                                       GetMediaTime(), AudioQueue(), VideoQueue());
   } else {
-    MOZ_ASSERT(false, "Cannot handle this seek task.");
+    // Use MOZ_DIAGNOSTIC_ASSERT here to test if a "VideoOnly" seek task could
+    // reach here, may come from a dormant state. Once we confirm it, we could
+    // than handle it.
+    MOZ_DIAGNOSTIC_ASSERT(false, "Cannot handle this seek task.");
   }
 
   // Stop playback now to ensure that while we're outside the monitor