Have Document::timeline() / Document::transitionTimeline() return a reference

Source/core/animation/DocumentTimelineTest.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 257 matching lines @@
 
     platformTiming->expectNextFrameAction();
     updateClockAndService(4.98);
 }
 
 TEST_F(AnimationDocumentTimelineTest, PlayAfterDocumentDeref)
 {
     timing.iterationDuration = 2;
     timing.startDelay = 5;
 
-    timeline = document->timeline();
+    timeline = &document->timeline();
     element = nullptr;
     document = nullptr;
 
     RefPtr<Animation> anim = Animation::create(nullptr, nullptr, timing);
     // Test passes if this does not crash.
     timeline->play(anim.get());
 }
 
 TEST_F(AnimationDocumentTimelineTest, UsePlayerAfterTimelineDeref)
 {
     RefPtr<Player> player = timeline->createPlayer(0);
     timeline.clear();
     // Test passes if this does not crash.
     player->setStartTime(0);
 }
 
 }