Implement 1:N mapping from ios::ChromeBrowserState to TabModel.

ios/chrome/browser/tabs/tab_model_list.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_CHROME_BROWSER_TABS_TAB_MODEL_LIST_H_
+#define IOS_CHROME_BROWSER_TABS_TAB_MODEL_LIST_H_
+
+#import <Foundation/Foundation.h>
+
+// This file contains free functions to help maintain a 1:N relationship
+// between an ios::ChromeBrowserState and multiple TabModels.
+
+@class TabModel;
+
+namespace ios {
+class ChromeBrowserState;
+}
+
+// Registers |tab_model| as associated to |browser_state|. The object will
+// be retained until |UnregisterTabModelFromChromeBrowserState| is called.
+// It is an error if |tab_model is already registered as associated to
+// |browser_state|.
+void RegisterTabModelWithChromeBrowserState(
+    ios::ChromeBrowserState* browser_state,
+    TabModel* tab_model);
+
+// Unregisters the association between |tab_model| and |browser_state|.
+// It is an error if no such association exists.
+void UnregisterTabModelFromChromeBrowserState(
+    ios::ChromeBrowserState* browser_state,
+    TabModel* tab_model);
+
+// Returns the list of all TabModels associated with |browser_state|.
+NSArray<TabModel*>* GetTabModelsForChromeBrowserState(
+    ios::ChromeBrowserState* browser_state);
+
+#endif  // IOS_CHROME_BROWSER_TABS_TAB_MODEL_LIST_H_