Adds OnClickCanceled callback to views::Button

ui/views/controls/button/button.h

Index: ui/views/controls/button/button.h
diff --git a/ui/views/controls/button/button.h b/ui/views/controls/button/button.h
index 84ca047b145c2fb26d65db01a8473cc36098de28..9dbb4f1286f0cc30a6f223fa9afca8f74e91688f 100644
--- a/ui/views/controls/button/button.h
+++ b/ui/views/controls/button/button.h
@@ -69,6 +69,12 @@ class VIEWS_EXPORT Button : public View {
   // Cause the button to notify the listener that a click occurred.
   virtual void NotifyClick(const ui::Event& event);
 
+  // Called when a button gets released without triggering an action or when
+  // a NOTIFY_ON_PRESS button is pressed with a wrong mouse button.

[Peter Kasting, 2015/12/01 02:03:18]

It's not intuitive why we should fire "canceled" for NOTIFY_ON_PRESS with the
wrong mouse button.  Either don't do that, fire a separate notification than
"canceled", or explain here why this actually makes sense.

[varkha, 2015/12/01 19:23:29]

Done. Thinking a bit more we should not really need that.

+  // Note: This is only wired up for mouse button events and not gesture
+  // events.
+  virtual void OnClickCanceled(const ui::Event& event);
+
   // The button's listener. Notified when clicked.
   ButtonListener* listener_;