Fix user agent overridding with PlzNavigate.

content/browser/web_contents/web_contents_impl.cc

Index: content/browser/web_contents/web_contents_impl.cc
diff --git a/content/browser/web_contents/web_contents_impl.cc b/content/browser/web_contents/web_contents_impl.cc
index f8910b4bb042c1d641c669f7c47f640208a757d5..20fdfedf77e2be5dfdba49c2b84914cb77d0d272 100644
--- a/content/browser/web_contents/web_contents_impl.cc
+++ b/content/browser/web_contents/web_contents_impl.cc
@@ -3426,6 +3426,10 @@ bool WebContentsImpl::ShouldPreserveAbortedURLs() {
   return delegate_->ShouldPreserveAbortedURLs(this);
 }
 
+const std::string& WebContentsImpl::GetNavigationUserAgentOverride() {
+  return renderer_preferences_.user_agent_override;
+}
+
 void WebContentsImpl::DidNavigateMainFramePreCommit(
     bool navigation_is_within_page) {
   // Ensure fullscreen mode is exited before committing the navigation to a