[ios clean] Add placeholder for NTP bookmarks, chrome home and open tabs.

ios/clean/chrome/browser/ui/web_contents/web_contents_mediator.mm

 // 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.
 
 #import "ios/clean/chrome/browser/ui/web_contents/web_contents_mediator.h"
 
 #include "base/memory/ptr_util.h"
+#include "ios/chrome/browser/chrome_url_constants.h"
 #import "ios/clean/chrome/browser/ui/web_contents/web_contents_consumer.h"
 #import "ios/shared/chrome/browser/tabs/web_state_list.h"
 #import "ios/shared/chrome/browser/tabs/web_state_list_observer_bridge.h"
 #import "ios/web/public/navigation_manager.h"
 #include "ios/web/public/web_state/web_state.h"
 #include "ui/base/page_transition_types.h"
 #include "url/gurl.h"
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
 #error "This file requires ARC support."
@@ skipping 77 matching lines @@
     [self navigateToDefaultPage:webState];
   }
   if (self.consumer) {
     [self.consumer contentViewDidChange:updatedView];
   }
 }
 
 // PLACEHOLDER: This navigates the page since the omnibox is not yet hooked up.
 - (void)navigateToDefaultPage:(web::WebState*)webState {
   if (!webState->GetNavigationManager()->GetItemCount()) {
-    web::NavigationManager::WebLoadParams params(
-        GURL("https://dev.chromium.org/"));
+    web::NavigationManager::WebLoadParams params((GURL(kChromeUINewTabURL)));
     params.transition_type = ui::PAGE_TRANSITION_TYPED;
     webState->GetNavigationManager()->LoadURLWithParams(params);
   }
 }
 
 @end