blink: Make WebMainThreadScrollingReason wrap the cc enum.

third_party/WebKit/Source/platform/scroll/MainThreadScrollingReason.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 MainThreadScrollingReason_h
+#define MainThreadScrollingReason_h
+
+#include "cc/input/main_thread_scrolling_reason.h"
+
+namespace blink {
+
+// A wrapper around cc's structure to expose it to core.
+struct MainThreadScrollingReason : public cc::MainThreadScrollingReason {
+};
+
+} // namespace blink
+
+#endif