Bug 709747 - Support nsCOMPtr<T>::forget(I**); r=bsmedberg

xpcom/glue/nsCOMPtr.h

@@ -767,15 +767,17 @@ class nsCOMPtr MOZ_FINAL
           return temp;
         }
 
+      template <typename I>
       void
-      forget( T** rhs NS_OUTPARAM )
+      forget( I** rhs NS_OUTPARAM )
           // Set the target of rhs to the value of mRawPtr and null out mRawPtr.
           // Useful to avoid unnecessary AddRef/Release pairs with "out"
-          // parameters.
+          // parameters where rhs bay be a T** or an I** where I is a base class
+          // of T.
         {
           NS_ASSERTION(rhs, "Null pointer passed to forget!");
-          *rhs = 0;
-          swap(*rhs);
+          *rhs = get();
+          mRawPtr = 0;
         }
 
       T*