Update comments about not setting kHandlingScrollFromMainThread on subframes

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

Index: third_party/WebKit/Source/platform/scroll/ScrollAnimator.cpp
diff --git a/third_party/WebKit/Source/platform/scroll/ScrollAnimator.cpp b/third_party/WebKit/Source/platform/scroll/ScrollAnimator.cpp
index 4c27d32c64841bdf536dca0f063704c8790c73af..08005550e6fd50100c4ed0d035eff1ff51b5db1e 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollAnimator.cpp
+++ b/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(