Composited Animations: Support --disable-threaded-animation in new animation system.

Source/core/animation/AnimationTimeline.cpp

Index: Source/core/animation/AnimationTimeline.cpp
diff --git a/Source/core/animation/AnimationTimeline.cpp b/Source/core/animation/AnimationTimeline.cpp
index 28548a928b31314deb2e781a48c3269f7d3543fb..ad6bbaa002db6d386efeaddf970423f410d24e5b 100644
--- a/Source/core/animation/AnimationTimeline.cpp
+++ b/Source/core/animation/AnimationTimeline.cpp
@@ -77,8 +77,10 @@ AnimationTimeline::AnimationTimeline(Document* document, PassOwnPtrWillBeRawPtr<
     else
         m_timing = timing;
 
-    if (RuntimeEnabledFeatures::compositorAnimationTimelinesEnabled() && Platform::current()->compositorSupport())
+    if (RuntimeEnabledFeatures::compositorAnimationTimelinesEnabled() && Platform::current()->isThreadedAnimationEnabled()) {
+        ASSERT(Platform::current()->compositorSupport());
         m_compositorTimeline = adoptPtr(Platform::current()->compositorSupport()->createAnimationTimeline());
+    }
 
     ASSERT(document);
 }