Move logic of recording main thread scrolling reasons from cc to blink::ScrollManager

ui/events/blink/input_handler_proxy.cc

Index: ui/events/blink/input_handler_proxy.cc
diff --git a/ui/events/blink/input_handler_proxy.cc b/ui/events/blink/input_handler_proxy.cc
index ca73fa685c4c7e8d197a58dc4e41dff4accc6d45..b96479426f396c0058696a7b5708899ee2c5f236 100644
--- a/ui/events/blink/input_handler_proxy.cc
+++ b/ui/events/blink/input_handler_proxy.cc
@@ -557,6 +557,9 @@ void InputHandlerProxy::RecordMainThreadScrollingReasons(
     return;
   }
 
+  // NonCompositedScrollReasons should only be set on the main thread.
+  DCHECK(!cc::MainThreadScrollingReason::NonCompositedScrollReasons(reasons));

[bokan, 2017/04/07 17:39:03]

I think this check isn't what you want. The function currently currently means
(has any reason && all reasons are non-composited) so you're checking:

!(has any reason) || !(all reasons are non-composited)

Which is equivalent to:

No reasons || has some composited reasons

See my comment in NonCompositedScrollReasons() for how to address this.

[yigu, 2017/04/07 19:26:27]

Done.

+
   // UMA_HISTOGRAM_ENUMERATION requires that the enum_max must be strictly
   // greater than the sample value. kMainThreadScrollingReasonCount doesn't
   // include the NotScrollingOnMain enum but the histograms do so adding