[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"
+
+#include "base/logging.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
+
+namespace {
+int NavigationIndexFromWebState(web::WebState* web_state) {
+  if (!web_state)
+    return -1;
+
+  DCHECK(web_state->GetNavigationManager());
+  return web_state->GetNavigationManager()->GetLastCommittedItemIndex();
+}
+}  // namespace
+
+WebStateOpener::WebStateOpener(web::WebState* opener)
+    : WebStateOpener(opener, NavigationIndexFromWebState(opener)) {}
+
+WebStateOpener::WebStateOpener(web::WebState* opener, int navigation_index)
+    : opener(opener), navigation_index(navigation_index) {}