[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..cbed4c3ffdb8c4ae3bc6d16c1e5f0ce70d681a13 100644
--- a/ui/views/cocoa/bridged_content_view_touch_bar.mm
+++ b/ui/views/cocoa/bridged_content_view_touch_bar.mm
@@ -61,6 +61,8 @@ NSString* const kTouchBarCancelId = @"com.google.chrome-CANCEL";
     }
     if ([items count] == 0)
       return nil;
+    static Class NSGroupTouchBarItem =

[tapted, 2016/11/19 07:27:38]

nit: no static (although the ban on non-POD static doesn't apply, this will
still take up space in the data segment)

+        NSClassFromString(@"NSGroupTouchBarItem");
     return [NSGroupTouchBarItem groupItemWithIdentifier:identifier items:items];
   }