[tab_grid] Hack-in new tab FAB action.

ios/clean/chrome/browser/ui/tab_grid/tab_grid_view_controller.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.
 
 // ======                        New Architecture                         =====
 // =         This code is only used in the new iOS Chrome architecture.       =
 // ============================================================================
 
 #import "ios/clean/chrome/browser/ui/tab_grid/tab_grid_view_controller.h"
 
@@ skipping 127 matching lines @@
 - (void)showSettings:(id)sender {
   [self.settingsCommandHandler showSettings];
 }
 
 #pragma mark - TabGridActions
 
 - (void)showTabGrid:(id)sender {
   [self.tabGridCommandHandler showTabGrid];
 }
 
+- (void)createNewTab:(id)sender {
+  // PLACEHOLDER: The new WebStateList data structure will have implications
+  // on how new tabs are created.
+  NSInteger index = [self.grid numberOfItemsInSection:0];
+  NSIndexPath* indexPath = [NSIndexPath indexPathForItem:index inSection:0];
+  auto updateBlock = ^{
+    [self.tabCommandHandler createNewTabAtIndexPath:indexPath];
+    [self.tabCommandHandler showTabAtIndexPath:indexPath];
+    [self.grid insertItemsAtIndexPaths:@[ indexPath ]];
+  };
+  [self.grid performBatchUpdates:updateBlock completion:nil];
+}
+
 #pragma mark - SessionCellDelegate
 
 - (TabSwitcherCache*)tabSwitcherCache {
   // PLACEHOLDER: return image cache.
   return nil;
 }
 
 - (void)cellPressed:(UICollectionViewCell*)cell {
   [self.tabCommandHandler showTabAtIndexPath:[self.grid indexPathForCell:cell]];
 }
 
 - (void)deleteButtonPressedForCell:(UICollectionViewCell*)cell {
   auto updateBlock = ^{
     NSIndexPath* indexPath = [self.grid indexPathForCell:cell];
     [self.tabCommandHandler closeTabAtIndexPath:indexPath];
     [self.grid deleteItemsAtIndexPaths:@[ indexPath ]];
   };
   [self.grid performBatchUpdates:updateBlock completion:nil];
 }
 
 @end