Add a skeleton Previews Prober

This utility class is the basic shape of the end result, documented in
the bug. There's several members and arguments which aren't used yet,
but will be in future CLs.

This CL adds a simple URLLoader and status checker. The prober's
behavior will become richer in future CLs.

Bug: 977603
Change-Id: Iba9c15c27d83012e357a793c5ecabd3fe4e94e71
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1671487
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Reviewed-by: Ramin Halavati <rhalavati@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672169}
6 files changed