Update comments about not setting kHandlingScrollFromMainThread on subframes

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

 /*
  * Copyright (c) 2011, 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 343 matching lines @@
 
     // Main thread should deal with the scroll animations it started.
     if (sentToCompositor || runningOnMainThread)
       addMainThreadScrollingReason();
     else
       removeMainThreadScrollingReason();
   }
 }
 
 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(
         MainThreadScrollingReason::kHandlingScrollFromMainThread);
   }
 }
 
 void ScrollAnimator::removeMainThreadScrollingReason() {
   if (WebLayer* scrollLayer =
           toWebLayer(getScrollableArea()->layerForScrolling())) {
@@ skipping 58 matching lines @@
     return false;
   }
   return true;
 }
 
 DEFINE_TRACE(ScrollAnimator) {
   ScrollAnimatorBase::trace(visitor);
 }
 
 }  // namespace blink