[ios] Use WebStateList to back the tab grid.

ios/showcase/tab_grid/sc_tab_grid_coordinator.mm

 // 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.
 
 #import "ios/showcase/tab_grid/sc_tab_grid_coordinator.h"
 
 #import "base/format_macros.h"
 #import "ios/clean/chrome/browser/ui/commands/tab_commands.h"
 #import "ios/clean/chrome/browser/ui/tab_grid/tab_grid_view_controller.h"
 #import "ios/showcase/common/protocol_alerter.h"
@@ skipping 22 matching lines @@
   self.viewController.dataSource = self;
   self.viewController.tabCommandHandler =
       static_cast<id<TabCommands>>(self.alerter);
 
   [self.baseViewController setHidesBarsOnSwipe:YES];
   [self.baseViewController pushViewController:self.viewController animated:YES];
 }
 
 #pragma mark - TabGridDataSource
 
-- (NSUInteger)numberOfTabsInTabGrid {
+- (int)numberOfTabsInTabGrid {
   return 3;
 }
 
-- (NSString*)titleAtIndex:(NSInteger)index {
-  return [NSString stringWithFormat:@"Tab %" PRIdNS, index];
+- (NSString*)titleAtIndex:(int)index {
+  return [NSString stringWithFormat:@"Tab %d", index];
 }
 
-- (NSInteger)indexOfActiveTab {
-  return NSNotFound;
+- (int)indexOfActiveTab {
+  return kTabGridDataSourceInvalidIndex;
 }
 
 @end