Prevent gap jumping while seeking.

Sometimes, when seeking backwards, the seek takes long enough for the playhead
to erroneously detect a gap and gap jump back to its previous position.
This only seems to happen when casting, presumably due to the additional
delay from message-passing.
This prevents gap jumps from happening while the video is seeking, and it should
solve the issue.

Closes #1061

Change-Id: I363c69428d84029a7808229c9c0e18eb691dd7f8

lib/media/playhead.js

@@ -340,7 +340,7 @@ shaka.media.Playhead.prototype.onSeekingToStartTime_ = function() {
  * @private
  */
 shaka.media.Playhead.prototype.onPollGapJump_ = function() {
-  if (this.video_.readyState == 0)
+  if (this.video_.readyState == 0 || this.video_.seeking)
     return;
 
   // When the ready state changes, we have moved on, so we should fire the large