Fix NavigationHandle being thrown out and recreated (without all the state) after rapid navigations.

content/renderer/render_frame_impl.h

Index: content/renderer/render_frame_impl.h
diff --git a/content/renderer/render_frame_impl.h b/content/renderer/render_frame_impl.h
index 71fd95c83b6c2b5bde9314b9b6f1022cd79c420d..fc30e76fcf44817c2b4846e4ef052d2af85e93f5 100644
--- a/content/renderer/render_frame_impl.h
+++ b/content/renderer/render_frame_impl.h
@@ -1416,6 +1416,10 @@ class CONTENT_EXPORT RenderFrameImpl
   // AndroidOverlay routing token from the browser, if we have one yet.
   base::Optional<base::UnguessableToken> overlay_routing_token_;
 
+  // PlzNavigate: true after the beforeunload handler that proceeded ran, and
+  // before the next navigation has committed.
+  bool ran_before_unload_ = false;
+
   // Callbacks that we should call when we get a routing token.
   std::vector<media::RoutingTokenCallback> pending_routing_token_callbacks_;