Bug 1032848 followup: Add 'override' annotation to Notify() method in CanvasCaptureMediaStream.cpp. rs=ehsan

dom/media/CanvasCaptureMediaStream.cpp

@@ -226,7 +226,7 @@ public:
   }
 
   NS_IMETHODIMP
-  Notify(nsITimer* aTimer)
+  Notify(nsITimer* aTimer) override
   {
     nsresult rv = TakeSnapshot();
     if (NS_FAILED(rv)) {