[Background Fetch] Pass request bodies to the Download Service.

Bug: 774054
Change-Id: Ia1c45819af4f60d52d3b661efcbbcbd77c58a143
Reviewed-on: https://chromium-review.googlesource.com/c/1373825
Commit-Queue: Rayan Kanso <rayankans@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616286}
16 files changed