Fix button event on mouse events.

third_party/WebKit/Source/core/events/MouseEvent.h

Index: third_party/WebKit/Source/core/events/MouseEvent.h
diff --git a/third_party/WebKit/Source/core/events/MouseEvent.h b/third_party/WebKit/Source/core/events/MouseEvent.h
index 34d076b37e989190c9e989a5dd455120a149abf9..eec939b184365b9c182f7d28dbb9afca903c9c74 100644
--- a/third_party/WebKit/Source/core/events/MouseEvent.h
+++ b/third_party/WebKit/Source/core/events/MouseEvent.h
@@ -91,7 +91,7 @@ class CORE_EXPORT MouseEvent : public UIEventWithKeyState {
 
   // WinIE uses 1,4,2 for left/middle/right but not for click (just for
   // mousedown/up, maybe others), but we will match the standard DOM.
-  virtual short button() const { return button_ == -1 ? 0 : button_; }
+  virtual short button() const;
   unsigned short buttons() const { return buttons_; }
   bool ButtonDown() const { return button_ != -1; }
   EventTarget* relatedTarget() const { return related_target_.Get(); }