Bug 1309162 - part4 : remove checking for mPlayingBeforeSeek. r=baku

In previous patch, we removed UpdateAudioChannelPlayingState() from AddRemoveSelfReference(), now we
don't call UpdateAudioChannelPlayingState() during the seeking. So we don't need to afraid to stop
audio channel agent during the seeking.


MozReview-Commit-ID: GTuIRB24zlq

dom/html/HTMLMediaElement.cpp

@@ -930,11 +930,6 @@ private:
       return true;
     }
 
-    // If we are seeking, we consider it as playing
-    if (mOwner->mPlayingBeforeSeek) {
-      return true;
-    }
-
     // If we are playing an external stream.
     if (mOwner->mSrcAttrStream) {
       return true;