[Mac] Support for Touch Bar Customization

ui/base/cocoa/touch_bar_forward_declarations.mm

Index: ui/base/cocoa/touch_bar_forward_declarations.mm
diff --git a/ui/base/cocoa/touch_bar_forward_declarations.mm b/ui/base/cocoa/touch_bar_forward_declarations.mm
new file mode 100644
index 0000000000000000000000000000000000000000..1df5a9b8551e937a828803b3618a29c32f76ea80
--- /dev/null
+++ b/ui/base/cocoa/touch_bar_forward_declarations.mm
@@ -0,0 +1,13 @@
+// Copyright 2017 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.
+
+#include "ui/base/cocoa/touch_bar_forward_declarations.h"
+
+#if !defined(MAC_OS_X_VERSION_10_12_1) || \
+    MAC_OS_X_VERSION_MIN_REQUIRED < MAC_OS_X_VERSION_10_12_1
+NSString* const NSTouchBarItemIdentifierFixedSpaceSmall =
+    @"NSTouchBarItemIdentifierFixedSpaceSmall";
+NSString* const NSTouchBarItemIdentifierFlexibleSpace =
+    @"NSTouchBarItemIdentifierFlexibleSpace";
+#endif  // MAC_OS_X_VERSION_10_12_1