Fix user agent overridding with PlzNavigate.

content/browser/frame_host/navigator_delegate.h

Index: content/browser/frame_host/navigator_delegate.h
diff --git a/content/browser/frame_host/navigator_delegate.h b/content/browser/frame_host/navigator_delegate.h
index c3db7d188332668261784b124efbf04e29fe140a..b60cfaabf1fc587c47296fbe45413684528b2a50 100644
--- a/content/browser/frame_host/navigator_delegate.h
+++ b/content/browser/frame_host/navigator_delegate.h
@@ -105,6 +105,9 @@ class CONTENT_EXPORT NavigatorDelegate {
   // preserved in the omnibox.  Defaults to false.
   virtual bool ShouldPreserveAbortedURLs();
 
+  // Returns the overriden user agent string if it's set.
+  virtual const std::string& GetUserAgentOverride() const = 0;
+
   // A RenderFrameHost in the specified |frame_tree_node| started loading a new
   // document. This correponds to Blink's notion of the throbber starting.
   // |to_different_document| will be true unless the load is a fragment