Implement cancel() API on a Notification object so that script can cancel or ...

chrome/browser/notifications/balloon_collection.h

Index: chrome/browser/notifications/balloon_collection.h
===================================================================
--- chrome/browser/notifications/balloon_collection.h	(revision 30967)
+++ chrome/browser/notifications/balloon_collection.h	(working copy)
@@ -26,6 +26,10 @@
   virtual void Add(const Notification& notification,
                    Profile* profile) = 0;
 
+  // Removes a balloon from the collection if present.  Returns
+  // true if anything was removed.
+  virtual bool Remove(const Notification& notification) = 0;
+
   // Is there room to add another notification?
   virtual bool HasSpace() const = 0;
 };
@@ -59,6 +63,7 @@
   // BalloonCollectionInterface overrides
   virtual void Add(const Notification& notification,
                    Profile* profile);
+  virtual bool Remove(const Notification& notification);
   virtual bool HasSpace() const;
 
   // Balloon::BalloonCloseListener interface