[ios] Use web_state_list in web_contents_mediator.

ios/clean/chrome/browser/ui/tab_collection/tab_collection_mediator.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_CLEAN_CHROME_BROWSER_UI_TAB_COLLECTION_TAB_COLLECTION_MEDIATOR_H_
 #define IOS_CLEAN_CHROME_BROWSER_UI_TAB_COLLECTION_TAB_COLLECTION_MEDIATOR_H_
 
+#import <Foundation/Foundation.h>
+
 #import "ios/clean/chrome/browser/ui/tab_collection/tab_collection_data_source.h"
 #import "ios/shared/chrome/browser/tabs/web_state_list_observer_bridge.h"
 
 @protocol TabCollectionConsumer;
 class WebStateList;
 
 // Mediator listens for web state list changes, then updates the consumer.
 // This also serves as data source for a tab collection.
 @interface TabCollectionMediator
     : NSObject<TabCollectionDataSource, WebStateListObserving>
 
 // The source of changes and backing of the data source.
 @property(nonatomic, assign) WebStateList* webStateList;
 
 // This consumer is updated whenever there are relevant changes to the web state
 // list.
 @property(nonatomic, weak) id<TabCollectionConsumer> consumer;
 
 @end
 
 #endif  // IOS_CLEAN_CHROME_BROWSER_UI_TAB_COLLECTION_TAB_COLLECTION_MEDIATOR_H_