GitLab

Loads the original page on a 404 or 503 server response. Further actions
for these response types will be in follow up CLs.

Bug: 864656
Change-Id: I861685e0d19f5597ade3ed814e9847a3595d9e31
Reviewed-on: https://chromium-review.googlesource.com/1171799
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583424}