Animations: Fix bug where getAnimationPlayers() crashes

ElementAnimation:getAnimationPlayers() crashes when it is called and no
animations have been previously created on the element.

BUG=396376

Review URL: https://codereview.chromium.org/423763002

git-svn-id: svn://svn.chromium.org/blink/trunk@179001 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/LayoutTests/web-animations-api/w3c/getAnimationPlayers.html

@@ -21,6 +21,8 @@ var element = document.getElementById('element');
 
 test(function() {
     assert_equals(document.timeline.getAnimationPlayers().length, 0);
+    assert_equals(container.getAnimationPlayers().length, 0);
+    assert_equals(element.getAnimationPlayers().length, 0);
 
     var player = element.animate([], 1000);
     assert_equals(document.timeline.getAnimationPlayers().length, 1);

third_party/WebKit/Source/core/animation/ElementAnimation.h

@@ -91,7 +91,11 @@ public:
 
     static WillBeHeapVector<RefPtrWillBeMember<AnimationPlayer> > getAnimationPlayers(Element& element)
     {
-        WillBeHeapVector<RefPtrWillBeMember<AnimationPlayer> > animationPlayers = WillBeHeapVector<RefPtrWillBeMember<AnimationPlayer> >();
+        WillBeHeapVector<RefPtrWillBeMember<AnimationPlayer> > animationPlayers;
+
+        if (!element.hasActiveAnimations())
+            return animationPlayers;
+
         const AnimationPlayerCountedSet& players = element.activeAnimations()->players();
 
         for (AnimationPlayerCountedSet::const_iterator it = players.begin(); it != players.end(); ++it) {