[Background Fetch] Create a cache entry handler for BGF.

The handler uses the side data blob to store the request body if any.

Bug: 774054
Change-Id: I14ec3e992543937bef7a601bcb603103842c6acf
Reviewed-on: https://chromium-review.googlesource.com/c/1363205
Commit-Queue: Rayan Kanso <rayankans@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Reviewed-by: Ben Kelly <wanderview@chromium.org>
Reviewed-by: Mugdha Lakhani <nator@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615515}
8 files changed