Fix user agent overridding with PlzNavigate.

content/browser/web_contents/web_contents_impl.h

 // 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.
 
 #ifndef CONTENT_BROWSER_WEB_CONTENTS_WEB_CONTENTS_IMPL_H_
 #define CONTENT_BROWSER_WEB_CONTENTS_WEB_CONTENTS_IMPL_H_
 
 #include <stdint.h>
 
 #include <functional>
@@ skipping 585 matching lines @@
       RenderFrameHostImpl* render_frame_host,
       const LoadCommittedDetails& details,
       const FrameHostMsg_DidCommitProvisionalLoad_Params& params) override;
   void SetMainFrameMimeType(const std::string& mime_type) override;
   bool CanOverscrollContent() const override;
   void NotifyChangedNavigationState(InvalidateTypes changed_flags) override;
   void DidStartNavigationToPendingEntry(const GURL& url,
                                         ReloadType reload_type) override;
   bool ShouldTransferNavigation(bool is_main_frame_navigation) override;
   bool ShouldPreserveAbortedURLs() override;
+  const std::string& GetNavigationUserAgentOverride() override;
   void DidStartLoading(FrameTreeNode* frame_tree_node,
                        bool to_different_document) override;
   void DidStopLoading() override;
   void DidChangeLoadProgress() override;
   std::vector<std::unique_ptr<NavigationThrottle>> CreateThrottlesForNavigation(
       NavigationHandle* navigation_handle) override;
   std::unique_ptr<NavigationUIData> GetNavigationUIData(
       NavigationHandle* navigation_handle) override;
 
   // RenderWidgetHostDelegate --------------------------------------------------
@@ skipping 927 matching lines @@
   // Adds/removes a callback called on creation of each new WebContents.
   static void AddCreatedCallbackForTesting(const CreatedCallback& callback);
   static void RemoveCreatedCallbackForTesting(const CreatedCallback& callback);
 
   DISALLOW_COPY_AND_ASSIGN(FriendZone);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_WEB_CONTENTS_WEB_CONTENTS_IMPL_H_