[ios] Use WebStateList to back the tab grid.

ios/clean/chrome/browser/ui/tab_grid/tab_grid_view_controller.h

 // Copyright 2016 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.
 
 // ======                        New Architecture                         =====
 // =         This code is only used in the new iOS Chrome architecture.       =
 // ============================================================================
 
 #ifndef IOS_CLEAN_CHROME_BROWSER_UI_TAB_GRID_TAB_GRID_VIEW_CONTROLLER_H_
 #define IOS_CLEAN_CHROME_BROWSER_UI_TAB_GRID_TAB_GRID_VIEW_CONTROLLER_H_
 
 #import <UIKit/UIKit.h>
 
 #import "ios/clean/chrome/browser/ui/animators/zoom_transition_delegate.h"
 
+const int kTabGridDataSourceInvalidIndex = -1;
+
 @protocol SettingsCommands;
 @protocol TabCommands;
 @protocol TabGridCommands;
 
 // The data source for tab grid UI.
 // Conceptually the tab grid represents a group of WebState objects (which
 // are ultimately the model-layer representation of a browser tab). The data
 // source must be able to map between indices and WebStates.
 @protocol TabGridDataSource<NSObject>
 
 // The number of tabs to be displayed in the grid.
-- (NSUInteger)numberOfTabsInTabGrid;
+- (int)numberOfTabsInTabGrid;
 
 // Title for the tab at |index| in the grid.
-- (NSString*)titleAtIndex:(NSInteger)index;
+- (NSString*)titleAtIndex:(int)index;
 
-// Index for the active tab.
-- (NSInteger)indexOfActiveTab;
+// Index for the active tab or kTabGridDataSourceInvalidIndex if there is no
+// active tab.
+- (int)indexOfActiveTab;
 
 @end
 
 // Controller for a scrolling view displaying square cells that represent
 // the user's open tabs.
 @interface TabGridViewController : UIViewController<ZoomTransitionDelegate>
 
 // Data source for the tabs to be displayed.
 @property(nonatomic, weak) id<TabGridDataSource> dataSource;
 // Command handlers.
 @property(nonatomic, weak) id<SettingsCommands> settingsCommandHandler;
 @property(nonatomic, weak) id<TabCommands> tabCommandHandler;
 @property(nonatomic, weak) id<TabGridCommands> tabGridCommandHandler;
 @end
 
 #endif  // IOS_CLEAN_CHROME_BROWSER_UI_TAB_GRID_TAB_GRID_VIEW_CONTROLLER_H_