Update NavigationHandleImpl#GetResponseHeaders method.

content/browser/frame_host/navigation_handle_impl.h

 // Copyright 2015 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.
 
 #ifndef CONTENT_BROWSER_FRAME_HOST_NAVIGATION_HANDLE_IMPL_H_
 #define CONTENT_BROWSER_FRAME_HOST_NAVIGATION_HANDLE_IMPL_H_
 
 #include "content/public/browser/navigation_handle.h"
 
 #include <stddef.h>
@@ skipping 88 matching lines @@
       ui::PageTransition transition,
       bool is_external_protocol) override;
   NavigationThrottle::ThrottleCheckResult CallWillRedirectRequestForTesting(
       const GURL& new_url,
       bool new_method_is_post,
       const GURL& new_referrer_url,
       bool new_is_external_protocol) override;
 
   NavigatorDelegate* GetDelegate() const;
 
-  // Returns the response headers for the request. This can only be accessed
-  // after a redirect was encountered or after the the navigation is ready to
-  // commit. It should not be modified, as modifications will not be reflected
-  // in the network stack.
+  // Returns the response headers for the request or nullptr if there are none.
+  // This should only be accessed after a redirect was encountered or after the
+  // navigation is ready to commit. The headers returned should not be modified,
+  // as modifications will not be reflected in the network stack.
   const net::HttpResponseHeaders* GetResponseHeaders();
 
   void set_net_error_code(net::Error net_error_code) {
     net_error_code_ = net_error_code;
   }
 
   // Returns whether the navigation is currently being transferred from one
   // RenderFrameHost to another. In particular, a DidStartProvisionalLoad IPC
   // for the navigation URL, received in the new RenderFrameHost, should not
   // indicate the start of a new navigation in that case.
@@ skipping 143 matching lines @@
   // Manages the lifetime of a pre-created ServiceWorkerProviderHost until a
   // corresponding ServiceWorkerNetworkProvider is created in the renderer.
   scoped_ptr<ServiceWorkerNavigationHandle> service_worker_handle_;
 
   DISALLOW_COPY_AND_ASSIGN(NavigationHandleImpl);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_FRAME_HOST_NAVIGATION_HANDLE_IMPL_H_