[ios] Add a delegate to WebStateList class.

ios/shared/chrome/browser/tabs/web_state_list_delegate.h

Index: ios/shared/chrome/browser/tabs/web_state_list_delegate.h
diff --git a/ios/shared/chrome/browser/tabs/web_state_list_delegate.h b/ios/shared/chrome/browser/tabs/web_state_list_delegate.h
new file mode 100644
index 0000000000000000000000000000000000000000..c1968bb1d6a474ed1b66b0723f52fa5b576cd93e
--- /dev/null
+++ b/ios/shared/chrome/browser/tabs/web_state_list_delegate.h
@@ -0,0 +1,30 @@
+// 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_DELEGATE_H_
+#define IOS_SHARED_CHROME_BROWSER_TABS_WEB_STATE_LIST_DELEGATE_H_
+
+#include "base/macros.h"
+
+namespace web {
+class WebState;
+}
+
+// A delegate interface that the WebStateList uses to perform work that it
+// cannot do itself such as attaching tab helpers, ...
+class WebStateListDelegate {
+ public:
+  WebStateListDelegate() = default;
+  virtual ~WebStateListDelegate() = default;
+
+  // Notifies the delegate that the specified WebState will be added to the
+  // WebStateList (via insertion/appending/replacing existing) and allows it
+  // to do any preparation that it deems necessary.
+  virtual void WillAddWebState(web::WebState* web_state) = 0;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(WebStateListDelegate);
+};
+
+#endif  // IOS_SHARED_CHROME_BROWSER_TABS_WEB_STATE_LIST_DELEGATE_H_