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

Source/web/WebViewImpl.cpp

Index: Source/web/WebViewImpl.cpp
diff --git a/Source/web/WebViewImpl.cpp b/Source/web/WebViewImpl.cpp
index ec12fcb9ef8e0eadf3bdde9ba53be5d6895ec921..719e3c4953e0dc505c2972504cd2c26d937b1ea2 100644
--- a/Source/web/WebViewImpl.cpp
+++ b/Source/web/WebViewImpl.cpp
@@ -4188,7 +4188,7 @@ void WebViewImpl::initializeLayerTreeView()
     // make this assert necessary. We should make them not hit this code and then delete allowsBrokenNullLayerTreeView.
     ASSERT(m_layerTreeView || !m_client || m_client->allowsBrokenNullLayerTreeView());
 
-    if (RuntimeEnabledFeatures::compositorAnimationTimelinesEnabled() && m_layerTreeView) {
+    if (RuntimeEnabledFeatures::compositorAnimationTimelinesEnabled() && Platform::current()->isThreadedAnimationEnabled() && m_layerTreeView) {
         ASSERT(Platform::current()->compositorSupport());
         m_linkHighlightsTimeline = adoptPtr(Platform::current()->compositorSupport()->createAnimationTimeline());
         attachCompositorAnimationTimeline(m_linkHighlightsTimeline.get());