Enforce marking "override" for functions overriding Blink.

content/renderer/webscrollbarbehavior_impl_gtkoraura.h

Index: content/renderer/webscrollbarbehavior_impl_gtkoraura.h
diff --git a/content/renderer/webscrollbarbehavior_impl_gtkoraura.h b/content/renderer/webscrollbarbehavior_impl_gtkoraura.h
index 30ccb239f37af450caf5e9c11c10e6893fdeb79a..3b0eb61905b3052410b37a17f849971758b5a445 100644
--- a/content/renderer/webscrollbarbehavior_impl_gtkoraura.h
+++ b/content/renderer/webscrollbarbehavior_impl_gtkoraura.h
@@ -11,13 +11,12 @@ 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