Update comments about not setting kHandlingScrollFromMainThread on subframes

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

Index: third_party/WebKit/Source/core/frame/FrameView.cpp
diff --git a/third_party/WebKit/Source/core/frame/FrameView.cpp b/third_party/WebKit/Source/core/frame/FrameView.cpp
index a8d40a8c5b11c1b2e3423c6fa5bcab5be896170c..2c52617c7a6c6092ba41bdc7386526ed2d2c3e47 100644
--- a/third_party/WebKit/Source/core/frame/FrameView.cpp
+++ b/third_party/WebKit/Source/core/frame/FrameView.cpp
@@ -4798,6 +4798,7 @@ void FrameView::updateSubFrameScrollOnMainReason(
       scrollLayer->clearMainThreadScrollingReasons(
           ~MainThreadScrollingReason::kHandlingScrollFromMainThread);
     }
+    reasons |= scrollLayer->mainThreadScrollingReasons();

[pdr., 2017/01/06 20:41:29]

Will updateSubFrameScrollOnMainReason always get called after
ScrollAnimator::addMainThreadScrollingReason and
ScrollAnimator::removeMainThreadScrollingReason? Can you add a unit test for
both of these cases?

kHandlingScrollFromMainThread is different from the other main thread scrolling
reasons. Could we unify them so all updates go through the same codepath?

[yigu, 2017/01/11 22:44:45]

As discussed, we will not update this reason on subframes.

   }
 
   Frame* child = frame.tree().firstChild();