Fix WebAuthFlow crash of dereferencing nullptr navigation_handle->GetResponseHeaders().

chrome/browser/extensions/api/identity/web_auth_flow.cc

 // Copyright (c) 2012 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.
 
 #include "chrome/browser/extensions/api/identity/web_auth_flow.h"
 
 #include <utility>
 
 #include "base/base64.h"
 #include "base/location.h"
@@ skipping 194 matching lines @@
 void WebAuthFlow::DidFinishNavigation(
     content::NavigationHandle* navigation_handle) {
   bool failed = false;
 
   if (navigation_handle->GetNetErrorCode() != net::OK) {
     failed = true;
     TRACE_EVENT_ASYNC_STEP_PAST1("identity", "WebAuthFlow", this,
                                  "DidFinishNavigationFailure", "error_code",
                                  navigation_handle->GetNetErrorCode());
   } else if (navigation_handle->IsInMainFrame() &&
+             navigation_handle->GetResponseHeaders() &&
              navigation_handle->GetResponseHeaders()->response_code() >= 400) {
     failed = true;
     TRACE_EVENT_ASYNC_STEP_PAST1(
         "identity", "WebAuthFlow", this, "DidFinishNavigationFailure",
         "response_code",
         navigation_handle->GetResponseHeaders()->response_code());
   }
 
   if (failed && delegate_)
     delegate_->OnAuthFlowFailure(LOAD_FAILED);
 }
 
 }  // namespace extensions