Initial framework setup and skeleton for BackgroundFetchContext

The Background Fetch system will be a service worker based mechanism for
allowing persistent and reliable uploads and downloads.

This CL sets up some of the very basic framework, including the directory
and BackgroundFetchContext, which will be the main dispatcher for the
service. Once this CL is in place, future CLs will quickly iterate
adding specific functionality.

BUG=679300

Review-Url: https://codereview.chromium.org/2678273003
Cr-Commit-Position: refs/heads/master@{#450785}
10 files changed