Make UMA_HISTOGRAM_ENUMERATION work with scoped enums.

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 20b84d1c4936211dbfaafe0c9ffb4c2fbecbbdee..c63e47366c2d240d24598d27a117bb1a0324c3ca 100644
--- a/ui/events/blink/input_handler_proxy.cc
+++ b/ui/events/blink/input_handler_proxy.cc
@@ -556,7 +556,8 @@ void InputHandlerProxy::RecordMainThreadScrollingReasons(
   // greater than the sample value. kMainThreadScrollingReasonCount doesn't
   // include the NotScrollingOnMain enum but the histograms do so adding
   // the +1 is necessary.
-  uint32_t mainThreadScrollingReasonEnumMax =
+  // TODO(dcheng): Fix https://crbug.com/705169 so this isn't needed.
+  constexpr uint32_t mainThreadScrollingReasonEnumMax =
       cc::MainThreadScrollingReason::kMainThreadScrollingReasonCount + 1;
   if (reasons == cc::MainThreadScrollingReason::kNotScrollingOnMain) {
     if (device == blink::WebGestureDeviceTouchscreen) {