[ios] Add a delegate to WebStateList class.

ios/chrome/browser/tabs/tab_model_web_state_list_delegate.h

Index: ios/chrome/browser/tabs/tab_model_web_state_list_delegate.h
diff --git a/ios/chrome/browser/tabs/tab_model_web_state_list_delegate.h b/ios/chrome/browser/tabs/tab_model_web_state_list_delegate.h
new file mode 100644
index 0000000000000000000000000000000000000000..6c8267ffe72854ecf41586e2835c278174d3735b
--- /dev/null
+++ b/ios/chrome/browser/tabs/tab_model_web_state_list_delegate.h
@@ -0,0 +1,24 @@
+// 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_CHROME_BROWSER_TABS_TAB_MODEL_WEB_STATE_LIST_DELEGATE_H_
+#define IOS_CHROME_BROWSER_TABS_TAB_MODEL_WEB_STATE_LIST_DELEGATE_H_
+
+#include "base/macros.h"
+#import "ios/shared/chrome/browser/tabs/web_state_list_delegate.h"
+
+// WebStateList delegate for the old architecture.
+class TabModelWebStateListDelegate : public WebStateListDelegate {
+ public:
+  TabModelWebStateListDelegate();
+  ~TabModelWebStateListDelegate() override;
+
+  // WebStateListDelegate implementation.
+  void WillAddWebState(web::WebState* web_state) override;

[sdefresne, 2017/03/13 17:15:30]

Upstream TabStripModelDelegate does not receive the TabStripModel so I don't
pass it either, but I'm wondering whether I should pass it. WDYT?

[rohitrao (ping after 24h), 2017/03/14 11:51:11]

I'm fine either way.  I think this kind of API should pass the model as an
argument, but we'll also never use it for anything.

+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(TabModelWebStateListDelegate);
+};
+
+#endif  // IOS_CHROME_BROWSER_TABS_TAB_MODEL_WEB_STATE_LIST_DELEGATE_H_