blink: Make WebMainThreadScrollingReason wrap the cc enum.

cc/blink/web_layer_impl.h

Index: cc/blink/web_layer_impl.h
diff --git a/cc/blink/web_layer_impl.h b/cc/blink/web_layer_impl.h
index 482c63343ad49c94228a58f2486920c37724dbf1..f63ba55fb5a430a00caa8aeda7d33cfc81009d2f 100644
--- a/cc/blink/web_layer_impl.h
+++ b/cc/blink/web_layer_impl.h
@@ -22,7 +22,6 @@
 #include "third_party/WebKit/public/platform/WebDoublePoint.h"
 #include "third_party/WebKit/public/platform/WebFloatPoint.h"
 #include "third_party/WebKit/public/platform/WebLayer.h"
-#include "third_party/WebKit/public/platform/WebMainThreadScrollingReason.h"
 #include "third_party/WebKit/public/platform/WebPoint.h"
 #include "third_party/WebKit/public/platform/WebRect.h"
 #include "third_party/WebKit/public/platform/WebSize.h"
@@ -130,8 +129,7 @@ class WebLayerImpl : public blink::WebLayer {
   void setHaveScrollEventHandlers(bool have_scroll_event_handlers) override;
   bool haveScrollEventHandlers() const override;
   void addMainThreadScrollingReasons(
-      blink::WebMainThreadScrollingReason::WebMainThreadScrollingReason
-          main_thread_scrolling_reasons) override;
+      uint32_t main_thread_scrolling_reasons) override;
   void clearMainThreadScrollingReasons() override;
   bool shouldScrollOnMainThread() const override;
   void setNonFastScrollableRegion(