Handling previously unaddressed fallback test

When navigating to a previews URL directly, the preview code (V1) falls
back to the original URL. In this case, it should not report any
pingback metrics. It is currently DCHECKing, but in release builds it
does report the pingback.

Bug: 934865
Change-Id: I4f1749d9ddeef49e98d3d2ee041669bdce232480
Reviewed-on: https://chromium-review.googlesource.com/c/1483941
Commit-Queue: Ryan Sturm <ryansturm@chromium.org>
Reviewed-by: Robert Ogden <robertogden@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634861}

chrome/browser/page_load_metrics/observers/previews_lite_page_redirect_metrics_observer.cc

@@ -36,7 +36,7 @@ PreviewsLitePageRedirectMetricsObserver::OnCommitCalled(
 
   previews::PreviewsUserData::ServerLitePageInfo* info =
       previews_data->server_lite_page_info();
-  if (!info)
+  if (!info || info->status == previews::ServerLitePageStatus::kUnknown)
     return STOP_OBSERVING;
 
   // Past this point, we know this navigation is a preview or at least attempted