Fix user agent overridding with PlzNavigate.

content/browser/frame_host/navigator_delegate.cc

Index: content/browser/frame_host/navigator_delegate.cc
diff --git a/content/browser/frame_host/navigator_delegate.cc b/content/browser/frame_host/navigator_delegate.cc
index 6b067c50371553e315e6be04d1dc3c8da257e5c7..c91260b2c0ed8f6a69ee5edf3f03a9a445135bed 100644
--- a/content/browser/frame_host/navigator_delegate.cc
+++ b/content/browser/frame_host/navigator_delegate.cc
@@ -4,6 +4,8 @@
 
 #include "content/browser/frame_host/navigator_delegate.h"
 
+#include "base/strings/string_util.h"
+
 namespace content {
 
 bool NavigatorDelegate::CanOverscrollContent() const {
@@ -19,6 +21,10 @@ bool NavigatorDelegate::ShouldPreserveAbortedURLs() {
   return false;
 }
 
+const std::string& NavigatorDelegate::GetNavigationUserAgentOverride() {
+  return base::EmptyString();
+}
+
 std::vector<std::unique_ptr<NavigationThrottle>>
 NavigatorDelegate::CreateThrottlesForNavigation(
     NavigationHandle* navigation_handle) {