Bug 1319706. Part 3 - remove the assertion that is no longer valid. r=kaku

MozReview-Commit-ID: FgmMTw7cy1w

--HG--
extra : rebase_source : 44ba27b2cb3cb555e03d424d7d5a3747cf198aee
extra : intermediate-source : f70d9634b839896110f2c90d30a00319b77e12d7
extra : source : c23e236ea2b3718e945b51fbf897ccae28a5c4a6

dom/media/MediaDecoderStateMachine.cpp

@@ -1440,8 +1440,6 @@ SeekingState::SeekCompleted()
   // Notify FirstFrameLoaded now if we haven't since we've decoded some data
   // for readyState to transition to HAVE_CURRENT_DATA and fire 'loadeddata'.
   if (!mMaster->mSentFirstFrameLoadedEvent) {
-    // Only MSE can start seeking before finishing decoding first frames.
-    MOZ_ASSERT(Reader()->ForceZeroStartTime());
     mMaster->FinishDecodeFirstFrame();
   }