When add main thread scrolling reason kHandlingScrolFromMainThread, there is...

When add main thread scrolling reason kHandlingScrolFromMainThread, there is no need to update the subframes as they are not scrollable when the reason is set to main frame.
BUG=675677

Review-Url: https://codereview.chromium.org/2620453003
Cr-Commit-Position: refs/heads/master@{#443228}

third_party/WebKit/Source/platform/scroll/ScrollAnimator.cpp

@@ -361,8 +361,11 @@ void ScrollAnimator::updateCompositorAnimations() {
 }
 
 void ScrollAnimator::addMainThreadScrollingReason() {
-  // TODO(yigu): Should update the frame tree to make sure
-  // the same reason is set for subframes. crbug.com/675677.
+  // Usually main thread scrolling reasons should be updated from
+  // one frame to all its descendants. khandlingScrollFromMainThread
+  // is a special case because its subframes cannot be scrolled
+  // when the reason is set. When the subframes are ready to scroll
+  // the reason has benn reset.
   if (WebLayer* scrollLayer =
           toWebLayer(getScrollableArea()->layerForScrolling())) {
     scrollLayer->addMainThreadScrollingReasons(