blink: Make WebMainThreadScrollingReason wrap the cc enum.

third_party/WebKit/public/platform/WebMainThreadScrollingReason.h

-// Copyright 2015 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-#ifndef WebMainThreadScrollingReason_h
-#define WebMainThreadScrollingReason_h
-
-#include "WebCommon.h"
-
-namespace blink {
-
-// Ensure this stays in sync with cc::MainThreadScrollingReason.
-namespace WebMainThreadScrollingReason {
-enum WebMainThreadScrollingReason {
-    NotScrollingOnMain = 0,
-    HasBackgroundAttachmentFixedObjects = 1 << 0,
-    HasNonLayerViewportConstrainedObjects = 1 << 1,
-    ThreadedScrollingDisabled = 1 << 2,
-    ScrollBarScrolling = 1 << 3,
-    PageOverlay = 1 << 4
-};
-} // namespace MainThreadScrollingReason
-
-} // namespace blink
-
-#endif