Add WebStateListOrderController to control WebState insertion.

ios/shared/chrome/browser/tabs/web_state_list_order_controller.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_list_order_controller.h"
+
+#include <cstdint>
+
+#include "base/logging.h"
+#import "ios/shared/chrome/browser/tabs/web_state_list.h"
+
+WebStateListOrderController::WebStateListOrderController(
+    WebStateList* web_state_list)
+    : web_state_list_(web_state_list) {
+  DCHECK(web_state_list_);
+}
+
+WebStateListOrderController::~WebStateListOrderController() = default;
+
+int WebStateListOrderController::DetermineInsertionIndex(
+    ui::PageTransition transition,
+    web::WebState* opener) const {
+  if (!opener)
+    return web_state_list_->count();
+
+  if (!PageTransitionCoreTypeIs(transition, ui::PAGE_TRANSITION_LINK))
+    return web_state_list_->count();
+
+  int opener_index = web_state_list_->GetIndexOfWebState(opener);
+  DCHECK_NE(WebStateList::kInvalidIndex, opener_index);
+
+  int list_child_index = web_state_list_->GetIndexOfLastWebStateOpenedBy(
+      opener, opener_index, true);
+
+  int reference_index = list_child_index != WebStateList::kInvalidIndex
+                            ? list_child_index
+                            : opener_index;
+
+  // Check for overflows (just a DCHECK as INT_MAX open WebState is unlikely).
+  DCHECK_LT(reference_index, INT_MAX);
+  return reference_index + 1;
+}