Mac: Fix compilation against the 10.12.1 SDK.

ui/base/cocoa/touch_bar_forward_declarations.h

 // 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.
 
 #ifndef UI_BASE_COCOA_TOUCH_BAR_FORWARD_DECLARATIONS_H_
 #define UI_BASE_COCOA_TOUCH_BAR_FORWARD_DECLARATIONS_H_
 
 // Once Chrome is built with the 10.12.1 SDK, the NSTouchBar… classes can be
 // referred to normally (instead of with NSClassFromString(@"…")), and this
 // file can be deleted.
@@ skipping 83 matching lines @@
 @protocol NSTouchBarDelegate<NSObject>
 
 @optional
 - (nullable NSTouchBarItem*)touchBar:(NSTouchBar*)touchBar
                makeItemForIdentifier:(NSTouchBarItemIdentifier)identifier
     NS_AVAILABLE_MAC(10_12_1);
 @end
 
 NS_ASSUME_NONNULL_END
 
+#elif MAC_OS_X_VERSION_MIN_REQUIRED < MAC_OS_X_VERSION_10_12_1
+
+// When compiling against the 10.12.1 SDK or later, just provide forward
+// declarations to suppress the partial availability warnings.
+@class NSCustomTouchBarItem;
+@class NSGroupTouchBarItem;
+@class NSTouchBar;
+@protocol NSTouchBarDelegate;
+@class NSTouchBarItem;
+
 #endif  // MAC_OS_X_VERSION_10_12_1
 
 #endif  // UI_BASE_COCOA_TOUCH_BAR_FORWARD_DECLARATIONS_H_