[Mac] Build fix when using newer 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 no longer supports OSX 10.12.0, this file can be deleted.
 #import <AppKit/AppKit.h>
 #import <Foundation/Foundation.h>
@@ skipping 75 matching lines @@
 
 @end
 
 @protocol NSTouchBarDelegate<NSObject>
 
 @optional
 - (nullable NSTouchBarItem*)touchBar:(NSTouchBar*)touchBar
                makeItemForIdentifier:(NSTouchBarItemIdentifier)identifier;
 @end
 
+#pragma clang assume_nonnull end

[spqchan, 2017/04/21 20:23:39]

Instead of putting this here, it makes more sense to move 

#pragma clang assume_nonnull begin

to before L14

+
 #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;
 
 @interface NSWindow (TouchBarSDK)
 @property(strong, readonly) NSTouchBar* touchBar;
 @end
 
 #endif  // MAC_OS_X_VERSION_10_12_1
 
 extern "C" {

[Robert Sesek, 2017/04/21 20:18:32]

This section should probably just move up to be near line 27, right?

 #if !defined(MAC_OS_X_VERSION_10_12_1) || \
     MAC_OS_X_VERSION_MIN_REQUIRED < MAC_OS_X_VERSION_10_12_1
 UI_BASE_EXPORT extern NSString* const NSTouchBarItemIdentifierFixedSpaceSmall;
 UI_BASE_EXPORT extern NSString* const NSTouchBarItemIdentifierFlexibleSpace;
 #endif  // MAC_OS_X_VERSION_10_12_1
 }  // extern "C"
 
-#pragma clang assume_nonnull end
 
 #endif  // UI_BASE_COCOA_TOUCH_BAR_FORWARD_DECLARATIONS_H_