[Mac] Support for Touch Bar Customization

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>
 
-#import <Foundation/Foundation.h>
+#include "ui/base/ui_base_export.h"
 
 #if !defined(MAC_OS_X_VERSION_10_12_1)
 
 #pragma clang assume_nonnull begin
 
 @class NSTouchBar, NSTouchBarItem;
 @protocol NSTouchBarDelegate;
 
 typedef float NSTouchBarItemPriority;
 static const NSTouchBarItemPriority NSTouchBarItemPriorityHigh = 1000;
 static const NSTouchBarItemPriority NSTouchBarItemPriorityNormal = 0;
 static const NSTouchBarItemPriority NSTouchBarItemPriorityLow = -1000;
 
 typedef NSString* NSTouchBarItemIdentifier;
 typedef NSString* NSTouchBarCustomizationIdentifier;
 
-static const NSTouchBarItemIdentifier NSTouchBarItemIdentifierFixedSpaceSmall =
-    @"NSTouchBarItemIdentifierFixedSpaceSmall";
-
-static const NSTouchBarItemIdentifier NSTouchBarItemIdentifierFlexibleSpace =
-    @"NSTouchBarItemIdentifierFlexibleSpace";
-
 @interface NSTouchBar : NSObject<NSCoding>
 
 - (instancetype)init NS_DESIGNATED_INITIALIZER;
 - (nullable instancetype)initWithCoder:(NSCoder*)aDecoder
     NS_DESIGNATED_INITIALIZER;
 
 @property(copy, nullable)
     NSTouchBarCustomizationIdentifier customizationIdentifier;
 @property(copy) NSArray* customizationAllowedItemIdentifiers;
 @property(copy) NSArray* customizationRequiredItemIdentifiers;
@@ skipping 47 matching lines @@
 
 @end
 
 @protocol NSTouchBarDelegate<NSObject>
 
 @optional
 - (nullable NSTouchBarItem*)touchBar:(NSTouchBar*)touchBar
                makeItemForIdentifier:(NSTouchBarItemIdentifier)identifier;
 @end
 
-#pragma clang 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;
 
 @interface NSWindow (TouchBarSDK)
 @property(strong, readonly) NSTouchBar* touchBar;
 @end
 
 #endif  // MAC_OS_X_VERSION_10_12_1
 
+extern "C" {
+#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_