[Mac] Support for Touch Bar Customization

ui/base/cocoa/touch_bar_forward_declarations.h

Index: ui/base/cocoa/touch_bar_forward_declarations.h
diff --git a/ui/base/cocoa/touch_bar_forward_declarations.h b/ui/base/cocoa/touch_bar_forward_declarations.h
index 0e33268053145de4a44b4bb8d36ea826527b59dd..ce03c6aae01fa624634aa51b475f948a18b9a7be 100644
--- a/ui/base/cocoa/touch_bar_forward_declarations.h
+++ b/ui/base/cocoa/touch_bar_forward_declarations.h
@@ -6,9 +6,11 @@
 #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>
 
+#include "ui/base/ui_base_export.h"
+
 #if !defined(MAC_OS_X_VERSION_10_12_1)
 
 #pragma clang assume_nonnull begin
@@ -24,12 +26,6 @@ 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;
@@ -97,8 +93,6 @@ static const NSTouchBarItemIdentifier NSTouchBarItemIdentifierFlexibleSpace =
                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
@@ -116,4 +110,14 @@ static const NSTouchBarItemIdentifier NSTouchBarItemIdentifierFlexibleSpace =
 
 #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_