[Background Fetch] Create DatabaseTask for context initialization.

Adds a new DatabaseTask to be used by BackgroundFetchContext upon
initialization. Gets all the required information to resume fetches from
the previous session.

TBR=avi@chromium.org

Bug: 853058
Change-Id: Ic31aa0d7794427d8d63ed46ed71c100025a3ccae
Reviewed-on: https://chromium-review.googlesource.com/1101683
Commit-Queue: Rayan Kanso <rayankans@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572794}
6 files changed