First programmatic smooth scroll should run on the compositor if it can

third_party/WebKit/Source/core/frame/VisualViewport.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 703 matching lines @@
     // A document is considered adapted to small screen UAs if one of these holds:
     // 1. The author specified viewport has a constrained width that is equal to
     //    the initial viewport width.
     // 2. The author has disabled viewport zoom.
     const PageScaleConstraints& constraints = frameHost().pageScaleConstraintsSet().pageDefinedConstraints();
 
     return mainFrame()->view()->layoutSize().width() == m_size.width()
         || (constraints.minimumScale == constraints.maximumScale && constraints.minimumScale != -1);
 }
 
+CompositorAnimationTimeline* VisualViewport::compositorAnimationTimeline() const
+{
+    ScrollingCoordinator* c = frameHost().page().scrollingCoordinator();
+    return c ? c->compositorAnimationTimeline() : nullptr;
+}
+
 String VisualViewport::debugName(const GraphicsLayer* graphicsLayer) const
 {
     String name;
     if (graphicsLayer == m_innerViewportContainerLayer.get()) {
         name = "Inner Viewport Container Layer";
     } else if (graphicsLayer == m_overscrollElasticityLayer.get()) {
         name =  "Overscroll Elasticity Layer";
     } else if (graphicsLayer == m_pageScaleLayer.get()) {
         name =  "Page Scale Layer";
     } else if (graphicsLayer == m_innerViewportScrollLayer.get()) {
         name =  "Inner Viewport Scroll Layer";
     } else if (graphicsLayer == m_overlayScrollbarHorizontal.get()) {
         name =  "Overlay Scrollbar Horizontal Layer";
     } else if (graphicsLayer == m_overlayScrollbarVertical.get()) {
         name =  "Overlay Scrollbar Vertical Layer";
     } else if (graphicsLayer == m_rootTransformLayer) {
         name =  "Root Transform Layer";
     } else {
         ASSERT_NOT_REACHED();
     }
 
     return name;
 }
 
 } // namespace blink