Mac: Fix SDK 10.11 Compile errors after clang roll in r387792

ui/views/cocoa/native_widget_mac_nswindow.mm

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "ui/views/cocoa/native_widget_mac_nswindow.h"
 
 #include "base/mac/foundation_util.h"
 #import "ui/views/cocoa/bridged_native_widget.h"
 #import "ui/base/cocoa/user_interface_item_command_handler.h"
 #import "ui/views/cocoa/views_nswindow_delegate.h"
@@ skipping 181 matching lines @@
 }
 
 // NSWindow overrides (NSUserInterfaceItemValidations implementation)
 
 - (BOOL)validateUserInterfaceItem:(id<NSValidatedUserInterfaceItem>)item {
   // Since this class implements these selectors, |super| will always say they
   // are enabled. Only use [super] to validate other selectors. If there is no
   // command handler, defer to AppController.
   if ([item action] == @selector(commandDispatch:) ||
       [item action] == @selector(commandDispatchUsingKeyModifiers:)) {
-    return commandHandler_
-               ? [commandHandler_ validateUserInterfaceItem:item window:self]
-               : [[NSApp delegate] validateUserInterfaceItem:item];
+    if (commandHandler_)
+      return [commandHandler_ validateUserInterfaceItem:item window:self];
+
+    id appController = [NSApp delegate];
+    DCHECK([appController
+        conformsToProtocol:@protocol(NSUserInterfaceValidations)]);
+    return [appController validateUserInterfaceItem:item];
   }
 
   return [super validateUserInterfaceItem:item];
 }
 
 @end