[ios] Remove CWV_IMPLEMENTATION hack in ChromeWebView implementation.

ios/web_view/public/cwv_user_content_controller.h

 // 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.
 
 #ifndef IOS_WEB_VIEW_PUBLIC_CWV_USER_CONTENT_CONTROLLER_H_
 #define IOS_WEB_VIEW_PUBLIC_CWV_USER_CONTENT_CONTROLLER_H_
 
 #import <Foundation/Foundation.h>
 
+#import "cwv_export.h"
+
 @class CWVUserScript;
 
 // Allows injecting custom scripts into CWVWebView created with the
 // configuration.
 @interface CWVUserContentController : NSObject
 
 // The user scripts associated with the configuration.
 @property(nonatomic, copy, readonly, nonnull)
     NSArray<CWVUserScript*>* userScripts;
 
 - (nonnull instancetype)init NS_UNAVAILABLE;
 
 // Adds a user script.
 - (void)addUserScript:(nonnull CWVUserScript*)userScript;
 
 // Removes all associated user scripts.
 - (void)removeAllUserScripts;
 
 @end
 
 #endif  // IOS_WEB_VIEW_PUBLIC_CWV_USER_CONTENT_CONTROLLER_H_