Fix user agent overridding with PlzNavigate.

content/browser/web_contents/web_contents_impl.h

Index: content/browser/web_contents/web_contents_impl.h
diff --git a/content/browser/web_contents/web_contents_impl.h b/content/browser/web_contents/web_contents_impl.h
index 9124e255c0150abf44f8bf9ab21a5e5957e49c9a..23bce1188724303de226d5cf4cad17ce2fc86768 100644
--- a/content/browser/web_contents/web_contents_impl.h
+++ b/content/browser/web_contents/web_contents_impl.h
@@ -603,6 +603,7 @@ class CONTENT_EXPORT WebContentsImpl
                                         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;