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

ios/shared/chrome/browser/tabs/web_state_opener.mm

+// 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.
+
+#import "ios/shared/chrome/browser/tabs/web_state_opener.h"
+
+#import "ios/web/public/navigation_manager.h"
+#import "ios/web/public/web_state/web_state.h"
+
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
+WebStateOpener::WebStateOpener(web::WebState* opener, int navigation_index)
+    : opener(opener), navigation_index(navigation_index) {
+  if (opener && navigation_index < 0) {
+    this->navigation_index =
+        opener->GetNavigationManager()->GetLastCommittedItemIndex();
+  }
+}