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Also drops extra/legacy server preview call for server LoFi (just use
use LitePage check for Flywheel CPAT based previews) and drops
is_server_preview parm from ShouldAllow* method.

Also ignores case of blacklist not being loaded if flagged to ignore
blacklist.

Change-Id: I5995a13180a06c592ad5d02a49e54e875faade31
Reviewed-on: https://chromium-review.googlesource.com/c/1331415Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Doug Arnett <dougarnett@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607303}