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

ios/shared/chrome/browser/tabs/web_state_list_observer_bridge.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_OBSERVER_BRIDGE_H_
+#define IOS_SHARED_CHROME_BROWSER_TABS_WEB_STATE_LIST_OBSERVER_BRIDGE_H_
+
+#import <Foundation/Foundation.h>
+
+#include "base/macros.h"
+#import "ios/shared/chrome/browser/tabs/web_state_list_observer.h"
+
+// Protocol that correspond to WebStateListObserver API. Allows registering
+// Objective-C objects to listen to WebStateList events.
+@protocol WebStateListObserver<NSObject>

[rohitrao (ping after 24h), 2017/02/14 13:01:26]

How does this compile?  You can have a protocol with the same name as a C++
class?

Can we change this to WebStateListObserving, to avoid confusion?

[marq (ping after 24h), 2017/02/14 13:11:48]

Protocols aren't types, so there's no compilation issue. I said this was OK, but
maybe WebStateListObserving is better.

[sdefresne, 2017/02/14 13:39:57]

Done.

+
+@optional
+
+// Invoked when a new WebState has been added to the WebStateList at the
+// specified index.
+- (void)webStateList:(WebStateList*)webStateList
+    webStateInserted:(web::WebState*)webState

[rohitrao (ping after 24h), 2017/02/13 20:54:29]

To match cocoa touch conventions, consider naming this method
"didInsertWebState" instead of "webStateInserted".  (Or willInsert, if that's
when the method is called.)

Also for all of the observer methods below.

[marq (ping after 24h), 2017/02/14 13:11:48]

Yeah, I should have caught this. +1.

[sdefresne, 2017/02/14 13:39:57]

Done.

+             atIndex:(NSUInteger)index;
+
+// Invoked when the WebState at the specified index is moved to another index.
+- (void)webStateList:(WebStateList*)webStateList
+       webStateMoved:(web::WebState*)webState
+           fromIndex:(NSUInteger)fromIndex
+             toIndex:(NSUInteger)toIndex;
+
+// Invoked when the WebState at the specified index is replaced by another
+// WebState.
+- (void)webStateList:(WebStateList*)webStateList
+    webStateReplaced:(web::WebState*)oldWebState

[rohitrao (ping after 24h), 2017/02/13 20:54:29]

didReplaceWebState:withWebState:

[sdefresne, 2017/02/14 13:39:57]

Done.

+          byWebState:(web::WebState*)newWebState
+             atIndex:(NSUInteger)index;
+
+// Invoked when the WebState at the specified index is being removed. The
+// WebState is still valid but is no longer in the WebStateList.
+- (void)webStateList:(WebStateList*)webStateList
+     webStateRemoved:(web::WebState*)webState
+             atIndex:(NSUInteger)index;
+
+@end
+
+// Observer that bridges WebStateList events to an Objective-C observer that
+// implements the WebStateListObserver protocol (the observer is owned).
+class WebStateListObserverBridge : public WebStateListObserver {
+ public:
+  explicit WebStateListObserverBridge(id<WebStateListObserver> observer);
+  ~WebStateListObserverBridge() override;
+
+ private:
+  // WebStateListObserver implementation.
+  void WebStateInsertedAt(WebStateList* web_state_list,
+                          web::WebState* web_state,
+                          size_t index) override;
+  void WebStateMoved(WebStateList* web_state_list,
+                     web::WebState* web_state,
+                     size_t from_index,
+                     size_t to_index) override;
+  void WebStateReplacedAt(WebStateList* web_state_list,
+                          web::WebState* old_web_state,
+                          web::WebState* new_web_state,
+                          size_t index) override;
+  void WebStateRemovedAt(WebStateList* web_state_list,
+                         web::WebState* web_state,
+                         size_t index) override;
+
+  id<WebStateListObserver> observer_;
+
+  DISALLOW_COPY_AND_ASSIGN(WebStateListObserverBridge);
+};
+
+#endif  // IOS_SHARED_CHROME_BROWSER_TABS_WEB_STATE_LIST_OBSERVER_BRIDGE_H_