Add Page Based Scroll main thread scrolling reason.

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 8511af6c0a77f90deab4248cc146e6231cf9606c..fe659b93f5665ae185bbd52b2fda22e63686811e 100644
--- a/ui/events/blink/input_handler_proxy.cc
+++ b/ui/events/blink/input_handler_proxy.cc
@@ -433,6 +433,9 @@ InputHandlerProxy::EventDisposition InputHandlerProxy::HandleMouseWheel(
     // TODO(jamesr): We don't properly handle scroll by page in the compositor
     // thread, so punt it to the main thread. http://crbug.com/236639
     result = DID_NOT_HANDLE;
+    RecordMainThreadScrollingReasons(wheel_event.type,
+                                     cc::InputHandler::PAGE_BASED_SCROLLING);
+
   } else if (!wheel_event.canScroll) {
     // Wheel events with |canScroll| == false will not trigger scrolling,
     // only event handlers.  Forward to the main thread.