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

ui/views/cocoa/bridged_content_view_touch_bar.mm

Index: ui/views/cocoa/bridged_content_view_touch_bar.mm
diff --git a/ui/views/cocoa/bridged_content_view_touch_bar.mm b/ui/views/cocoa/bridged_content_view_touch_bar.mm
index 109f4cdafa8fd930869e60f12e3e8038bfd22aa7..7ff0011e5b6e4ff9a0156ccec0d056ef5f1956f2 100644
--- a/ui/views/cocoa/bridged_content_view_touch_bar.mm
+++ b/ui/views/cocoa/bridged_content_view_touch_bar.mm
@@ -61,7 +61,9 @@ NSString* const kTouchBarCancelId = @"com.google.chrome-CANCEL";
     }
     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;