Fix user agent overridding with PlzNavigate.

content/browser/frame_host/interstitial_page_impl.cc

Index: content/browser/frame_host/interstitial_page_impl.cc
diff --git a/content/browser/frame_host/interstitial_page_impl.cc b/content/browser/frame_host/interstitial_page_impl.cc
index 1a04a52b11a5ee010ca25244f3944f36d98a8510..a8cd1becc40d2cf7ff1f959bca47dd8d3b888f7b 100644
--- a/content/browser/frame_host/interstitial_page_impl.cc
+++ b/content/browser/frame_host/interstitial_page_impl.cc
@@ -531,6 +531,10 @@ WebContents* InterstitialPageImpl::OpenURL(const OpenURLParams& params) {
   return nullptr;
 }
 
+const std::string& InterstitialPageImpl::GetUserAgentOverride() const {
+  return base::EmptyString();
+}
+
 RendererPreferences InterstitialPageImpl::GetRendererPrefs(
     BrowserContext* browser_context) const {
   delegate_->OverrideRendererPrefs(&renderer_preferences_);