Revert of Web Animations: Timeline should not advance during task execution

Source/core/animation/DocumentAnimations.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 73 matching lines @@
     return document.timeline().hasOutdatedAnimationPlayer();
 }
 
 void DocumentAnimations::startPendingAnimations(Document& document)
 {
     ASSERT(document.lifecycle().state() == DocumentLifecycle::CompositingClean);
     if (document.compositorPendingAnimations().startPendingAnimations()) {
         ASSERT(document.view());
         document.view()->scheduleAnimation();
     }
+
+    document.animationClock().unfreeze();
 }
 
 } // namespace WebCore