Migrate part of the BackgroundFetchJobController to the UI thread

One of its responsibilities is to dispatch the requests with the
download manager, which must only be used on the UI thread. We need to
meet that requirement. This also hooks up Start() in the context to
ensure layout tests also exercise this code path.

BUG=

Review-Url: https://codereview.chromium.org/2781623009
Cr-Commit-Position: refs/heads/master@{#460730}
5 files changed