Add a callback for cancelling the context menu.

The callback updates the |visible| property to NO so clients can
correctly determine if the context menu has been dismissed.

BUG=489121

Review URL: https://codereview.chromium.org/1144873002

Cr-Commit-Position: refs/heads/master@{#330562}

ui/base/ios/cru_context_menu_controller.mm

@@ -268,10 +268,13 @@ - (void)showWithHolder:(CRUContextMenuHolder*)menuHolder
   }];
 
   // Cancel button goes last, to match other browsers.
+  void (^cancelHandler)(UIAlertAction*) = ^(UIAlertAction* action) {
+    [weakSelf setVisible:NO];
+  };
   UIAlertAction* cancel_action =
       [UIAlertAction actionWithTitle:l10n_util::GetNSString(IDS_APP_CANCEL)
                                style:UIAlertActionStyleCancel
-                             handler:nil];
+                             handler:cancelHandler];
   [alert addAction:cancel_action];
 
   // Present sheet/popover using controller that is added to view hierarchy.