[tab_grid] Hack-in new tab FAB action.

ios/clean/chrome/browser/ui/tab_grid/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.
 
 // ======                        New Architecture                         =====
 // =         This code is only used in the new iOS Chrome architecture.       =
 // ============================================================================
 
 #import "ios/clean/chrome/browser/ui/tab_grid/tab_grid_coordinator.h"
 
@@ skipping 96 matching lines @@
   [tabCoordinator start];
   _activeWebStateIndex = index;
 }
 
 - (void)closeTabAtIndexPath:(NSIndexPath*)indexPath {
   size_t index = static_cast<size_t>(indexPath.item);
   DCHECK(index < _webStates.size());
   _webStates.erase(_webStates.begin() + index);
 }
 
+- (void)createNewTabAtIndexPath:(NSIndexPath*)indexPath {
+  web::WebState::CreateParams webStateCreateParams(self.browserState);
+  std::unique_ptr<web::WebState> webState =
+      web::WebState::Create(webStateCreateParams);
+  _webStates.push_back(std::move(webState));
+}
+
 #pragma mark - TabGridCommands
 
 - (void)showTabGrid {
   // This object should only ever have at most one child.
   DCHECK_LE(self.children.count, 1UL);
   BrowserCoordinator* child = [self.children anyObject];
   [child stop];
   [self removeChildCoordinator:child];
 }
 
@@ skipping 25 matching lines @@
   activeWebState->GetNavigationManager()->LoadURLWithParams(params);
   if (!self.children.count) {
     [self showTabAtIndexPath:[NSIndexPath
                                  indexPathForItem:static_cast<NSUInteger>(
                                                       _activeWebStateIndex)
                                         inSection:0]];
   }
 }
 
 @end