Add WebStateListOrderController to control WebState insertion.

ios/shared/chrome/browser/tabs/web_state_list_order_controller.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_LIST_ORDER_CONTROLLER_H_
+#define IOS_SHARED_CHROME_BROWSER_TABS_WEB_STATE_LIST_ORDER_CONTROLLER_H_
+
+#include "base/macros.h"
+#include "ui/base/page_transition_types.h"
+
+class WebStateList;
+
+namespace web {
+class WebState;
+}
+
+// WebStateListOrderController allows different types of ordering and
+// selection heuristics to be plugged into WebStateList.
+class WebStateListOrderController {
+ public:
+  explicit WebStateListOrderController(WebStateList* web_state_list);
+  ~WebStateListOrderController();
+
+  // Determines where to place a newly opened WebState by using the supplied
+  // transition, opener and background flag.
+  int DetermineInsertionIndex(ui::PageTransition transition,
+                              web::WebState* opener) const;
+
+ private:
+  WebStateList* web_state_list_;
+
+  DISALLOW_COPY_AND_ASSIGN(WebStateListOrderController);
+};
+
+#endif  // IOS_SHARED_CHROME_BROWSER_TABS_WEB_STATE_LIST_ORDER_CONTROLLER_H_