[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 "ios/chrome/browser/chrome_url_constants.h"
 #import "ios/clean/chrome/browser/ui/web_contents/web_contents_consumer.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."
 #endif
 
 @implementation WebContentsMediator
 @synthesize webState = _webState;
 @synthesize consumer = _consumer;
 
 - (void)setWebState:(web::WebState*)webState {
   if (_webState)
     _webState->SetWebUsageEnabled(false);
 
   _webState = webState;
   if (!self.webState)
     return;
 
   self.webState->SetWebUsageEnabled(true);
   if (!self.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;
     self.webState->GetNavigationManager()->LoadURLWithParams(params);
   }
   [self.consumer contentViewDidChange:self.webState->GetView()];
 }
 
 - (void)setConsumer:(id<WebContentsConsumer>)consumer {
   _consumer = consumer;
   if (self.webState)
     [self.consumer contentViewDidChange:self.webState->GetView()];
 }
 
 @end