[Mac] Switch from NSObject categories to forward declarations for Touch Bar support.

ui/views/cocoa/bridged_content_view_touch_bar.mm

 // Copyright 2016 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 "base/mac/scoped_nsobject.h"
 #import "base/mac/sdk_forward_declarations.h"
 #include "base/strings/sys_string_conversions.h"
 #import "ui/base/cocoa/touch_bar_forward_declarations.h"
 #include "ui/base/models/dialog_model.h"
 #import "ui/views/cocoa/bridged_content_view.h"
@@ skipping 43 matching lines @@
 
   if ([identifier isEqualToString:kTouchBarDialogButtonsGroupId]) {
     NSMutableArray* items = [NSMutableArray arrayWithCapacity:2];
     for (NSTouchBarItemIdentifier i in @[ kTouchBarCancelId, kTouchBarOKId ]) {
       NSTouchBarItem* item = [self touchBar:touchBar makeItemForIdentifier:i];
       if (item)
         [items addObject:item];
     }
     if ([items count] == 0)
       return nil;
-    return [NSGroupTouchBarItem groupItemWithIdentifier:identifier items:items];
+    return [NSClassFromString(@"NSGroupTouchBarItem")
+        groupItemWithIdentifier:identifier
+                          items:items];
   }
 
   ui::DialogButton type = ui::DIALOG_BUTTON_NONE;
   if ([identifier isEqualToString:kTouchBarOKId])
     type = ui::DIALOG_BUTTON_OK;
   else if ([identifier isEqualToString:kTouchBarCancelId])
     type = ui::DIALOG_BUTTON_CANCEL;
   else
     return nil;
 
@@ skipping 41 matching lines @@
 
   // Use a group rather than individual items so they can be centered together.
   [bar setDefaultItemIdentifiers:@[ kTouchBarDialogButtonsGroupId ]];
 
   // Setting the group as principal will center it in the TouchBar.
   [bar setPrincipalItemIdentifier:kTouchBarDialogButtonsGroupId];
   return bar.autorelease();
 }
 
 @end