Don't update main thread scroll reasons on a throttled frame.

third_party/WebKit/Source/core/frame/FrameView.cpp

 /*
  * Copyright (C) 1998, 1999 Torben Weis <weis@kde.org>
  *                     1999 Lars Knoll <knoll@kde.org>
  *                     1999 Antti Koivisto <koivisto@kde.org>
  *                     2000 Dirk Mueller <mueller@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  *           (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
@@ skipping 4870 matching lines @@
     const Frame& frame,
     MainThreadScrollingReasons parentReason) {
   MainThreadScrollingReasons reasons = parentReason;
 
   if (!page()->settings().getThreadedScrollingEnabled())
     reasons |= MainThreadScrollingReason::kThreadedScrollingDisabled;
 
   if (!frame.isLocalFrame())
     return;
 
-  if (!toLocalFrame(frame).view()->layerForScrolling())
+  FrameView& frameView = *toLocalFrame(frame).view();
+  if (frameView.shouldThrottleRendering())
+    return;
+  if (!frameView.layerForScrolling())
     return;
 
-  reasons |= toLocalFrame(frame).view()->mainThreadScrollingReasonsPerFrame();
+  reasons |= frameView.mainThreadScrollingReasonsPerFrame();
   if (GraphicsLayer* layerForScrolling = toLocalFrame(frame)
                                              .view()
                                              ->layoutViewportScrollableArea()
                                              ->layerForScrolling()) {
     if (WebLayer* platformLayerForScrolling =
             layerForScrolling->platformLayer()) {
       if (reasons) {
         platformLayerForScrolling->addMainThreadScrollingReasons(reasons);
       } else {
         // Clear all main thread scrolling reasons except the one that's set
@@ skipping 189 matching lines @@
     std::unique_ptr<CompositorAnimationTimeline> timeline) {
   m_animationTimeline = std::move(timeline);
 }
 
 void FrameView::setAnimationHost(
     std::unique_ptr<CompositorAnimationHost> host) {
   m_animationHost = std::move(host);
 }
 
 }  // namespace blink