[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 {
+  web::WebState* opener;

[marq (ping after 24h), 2017/03/24 09:35:27]

Comment on every field.

[sdefresne, 2017/03/24 10:08:10]

Done.

+  int navigation_index;
+
+  // Creates WebStateOpener initialising the members from the parameters. If
+  // |navigation_index| is negative, it will be initialised from |opener|.

[marq (ping after 24h), 2017/03/24 09:35:26]

I dislike magic behavior for some values of integers.

Can you have an explicit second constructor that just takes |opener| for this
instead? I realize that *functionally* what you have here does this, but I'd
rather it happened without the -1.

[sdefresne, 2017/03/24 10:08:10]

Done.

+  explicit WebStateOpener(web::WebState* opener, int navigation_index = -1);
+
+  // Forbids default construction.
+  WebStateOpener() = delete;
+};
+
+#endif  // IOS_SHARED_CHROME_BROWSER_TABS_WEB_STATE_OPENER_H_