[ios] Change API to inform WebStateList of opener-opened relationship.

ios/shared/chrome/browser/tabs/web_state_opener.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_OPENER_H_
+#define IOS_SHARED_CHROME_BROWSER_TABS_WEB_STATE_OPENER_H_
+
+namespace web {
+class WebState;
+}
+
+// Represents the opener of a WebState.
+struct WebStateOpener {
+  // WebState responsible for the creation of the new WebState. May be null if
+  // the WebState has no opener.
+  web::WebState* opener;
+
+  // Recorded value of the |opener| last committed navigation index when the
+  // WebState was open. Value is undefined if |opener| is null.
+  int navigation_index;
+
+  // Creates WebStateOpener initialising the members from |opener| (the
+  // |navigation_index| will be initialised from |opener|'s navigation
+  // manager if |opener| is not null).
+  explicit WebStateOpener(web::WebState* opener);
+
+  // Creates WebStateOpener initialising the members from the parameters.
+  WebStateOpener(web::WebState* opener, int navigation_index);
+
+  // Forbids default construction.
+  WebStateOpener() = delete;
+};
+
+#endif  // IOS_SHARED_CHROME_BROWSER_TABS_WEB_STATE_OPENER_H_