MessageBubble which does not require separate window and does not grab input on mouse press event.

views/widget/widget_gtk.h

Index: views/widget/widget_gtk.h
diff --git a/views/widget/widget_gtk.h b/views/widget/widget_gtk.h
index beb629f0cb5487633336f27a636f2c9ffd128731..8f7ca17259089dbcf02ef1c6cda5d5077f2a4e48 100644
--- a/views/widget/widget_gtk.h
+++ b/views/widget/widget_gtk.h
@@ -270,11 +270,15 @@ class WidgetGtk
   virtual bool ReleaseCaptureOnMouseReleased() { return true; }
 
   // Does a mouse grab on this widget.
-  void DoGrab();
+  virtual void DoGrab();
 
   // Releases a grab done by this widget.
   virtual void ReleaseGrab();
 
+  // Invoked when input grab is stolen by other GtkWidget in the same
+  // application.
+  virtual void HandleGrabBroke();
+
   // Are we a subclass of WindowGtk?
   bool is_window_;
 
@@ -316,9 +320,6 @@ class WidgetGtk
   // MakeIgnoreEvents has been invoked.
   void ConfigureWidgetForIgnoreEvents();
 
-  // TODO(sky): documentation
-  void HandleGrabBroke();
-
   // A utility function to draw a transparent background onto the |widget|.
   static void DrawTransparentBackground(GtkWidget* widget,
                                         GdkEventExpose* event);