When cancelling an animation, add missing timeline-detach check.

Source/core/animation/Animation.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 818 matching lines @@
     if (playStateInternal() == Idle)
         return;
 
     m_holdTime = currentTimeInternal();
     m_held = true;
     // TODO
     m_playState = Idle;
     m_startTime = nullValue();
     m_currentTimePending = false;
 
-    InspectorInstrumentation::didCancelAnimation(timeline()->document(), this);
+    if (timeline())
+        InspectorInstrumentation::didCancelAnimation(timeline()->document(), this);
 }
 
 void Animation::beginUpdatingState()
 {
     // Nested calls are not allowed!
     ASSERT(!m_stateIsBeingUpdated);
     m_stateIsBeingUpdated = true;
 }
 
 void Animation::endUpdatingState()
@@ skipping 173 matching lines @@
     visitor->trace(m_content);
     visitor->trace(m_timeline);
     visitor->trace(m_pendingFinishedEvent);
     visitor->trace(m_finishedPromise);
     visitor->trace(m_readyPromise);
     EventTargetWithInlineData::trace(visitor);
     ActiveDOMObject::trace(visitor);
 }
 
 } // namespace