Fix user agent overridding with PlzNavigate.

content/browser/frame_host/navigator_delegate.cc

 // Copyright 2014 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 "content/browser/frame_host/navigator_delegate.h"
 
+#include "base/strings/string_util.h"
+
 namespace content {
 
 bool NavigatorDelegate::CanOverscrollContent() const {
   return false;
 }
 
 bool NavigatorDelegate::ShouldTransferNavigation(
     bool is_main_frame_navigation) {
   return true;
 }
 
 bool NavigatorDelegate::ShouldPreserveAbortedURLs() {
   return false;
 }
 
+const std::string& NavigatorDelegate::GetNavigationUserAgentOverride() {
+  return base::EmptyString();
+}
+
 std::vector<std::unique_ptr<NavigationThrottle>>
 NavigatorDelegate::CreateThrottlesForNavigation(
     NavigationHandle* navigation_handle) {
   return std::vector<std::unique_ptr<NavigationThrottle>>();
 }
 
 std::unique_ptr<NavigationUIData> NavigatorDelegate::GetNavigationUIData(
     NavigationHandle* navigation_handle) {
   return nullptr;
 }
 
 }  // namespace content