Add a callback for cancelling the context menu.

ui/base/ios/cru_context_menu_controller.mm

Index: ui/base/ios/cru_context_menu_controller.mm
diff --git a/ui/base/ios/cru_context_menu_controller.mm b/ui/base/ios/cru_context_menu_controller.mm
index c81615c0d644fcd8af83e2e73247ea1d20d6789d..27ce9ee25808a5151507b0c5b16bc81cc109bc3a 100644
--- a/ui/base/ios/cru_context_menu_controller.mm
+++ b/ui/base/ios/cru_context_menu_controller.mm
@@ -268,10 +268,13 @@ CGFloat GetScreenHeight() {
   }];
 
   // 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.