[Background Fetch] Remove chromium specific WPT code

Also add an upload endpoint to test Background Fetch uploads.

Bug: 774054
Change-Id: I305e9f43b5d2e77a8e78e9e460e8398d771b03e0
Reviewed-on: https://chromium-review.googlesource.com/c/1341923
Commit-Queue: Rayan Kanso <rayankans@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609394}
6 files changed