[EoS] Add backoff support EoS url fetcher

Also make EoS url fetcher support immediate or background fetch with
different backoff policy.

Bug: 867488
Change-Id: If41abed1f85781cf84b9038f600e0a94c6c1524c
Reviewed-on: https://chromium-review.googlesource.com/c/1263053
Commit-Queue: Jian Li <jianli@chromium.org>
Reviewed-by: Justin DeWitt <dewittj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597231}
13 files changed