Enforce marking "override" for functions overriding Blink.

content/renderer/webscrollbarbehavior_impl_gtkoraura.h

 // Copyright 2014 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 CONTENT_RENDERER_WEBSCROLLBARBEHAVIOR_IMPL_GTKORAURA_H_
 #define CONTENT_RENDERER_WEBSCROLLBARBEHAVIOR_IMPL_GTKORAURA_H_
 
 #include "third_party/WebKit/public/platform/WebScrollbarBehavior.h"
 
 namespace content {
 
 class WebScrollbarBehaviorImpl : public blink::WebScrollbarBehavior {
  public:
-  virtual bool shouldCenterOnThumb(
-      blink::WebScrollbarBehavior::Button mouseButton,
-      bool shiftKeyPressed,
-      bool altKeyPressed);
-  virtual bool shouldSnapBackToDragOrigin(const blink::WebPoint& eventPoint,
-                                          const blink::WebRect& scrollbarRect,
-                                          bool isHorizontal);
+  bool shouldCenterOnThumb(blink::WebScrollbarBehavior::Button mouseButton,
+                           bool shiftKeyPressed,
+                           bool altKeyPressed) override;
+  bool shouldSnapBackToDragOrigin(const blink::WebPoint& eventPoint,
+                                  const blink::WebRect& scrollbarRect,
+                                  bool isHorizontal) override;
 };
 
 }  // namespace content
 
 #endif  // CONTENT_RENDERER_WEBSCROLLBARBEHAVIOR_IMPL_GTKORAURA_H_