Introduce WebStateList to manage a list of web::WebState.

ios/shared/chrome/browser/tabs/web_state_list_fast_enumeration_helper.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_SHARED_CHROME_BROWSER_TABS_WEB_STATE_LIST_FAST_ENUMERATION_HELPER_H_
+#define IOS_SHARED_CHROME_BROWSER_TABS_WEB_STATE_LIST_FAST_ENUMERATION_HELPER_H_
+
+#import <Foundation/Foundation.h>
+
+class WebStateList;
+
+namespace web {
+class WebState;
+}
+
+// Protocol used to get Objective-C proxies for WebStates.
+@protocol WebStateProxyFactory
+
+// Returns an Objective-C object corresponding to a given WebState.
+- (id)proxyForWebState:(web::WebState*)webState;
+
+@end
+
+// Helper class allowing NSFastEnumeration over a WebStateList.
+@interface WebStateListFastEnumerationHelper : NSObject<NSFastEnumeration>
+
+- (instancetype)initWithWebStateList:(WebStateList*)webStateList
+                        proxyFactory:(id<WebStateProxyFactory>)proxyFactory
+    NS_DESIGNATED_INITIALIZER;
+
+- (instancetype)init NS_UNAVAILABLE;
+
+@end
+
+#endif  // IOS_SHARED_CHROME_BROWSER_TABS_WEB_STATE_LIST_FAST_ENUMERATION_HELPER_H_