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This is needed to support multiple fallback hosts.

BUG=742666
TEST=Modified translator and validator unit tests to include
checks for ExtraHosts

Change-Id: I3faa20fb5d3f3ca490340527ad0a40a7fe9f837a
Reviewed-on: https://chromium-review.googlesource.com/720138Reviewed-by: Kevin Cernekee <cernekee@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Matthew Wang <matthewmwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509536}