Add URLLoaderFactory for loading resources from a WebBundle

In follow-up CLs, this factory will be created by LinkWebBundle, and
will be used to load subresources from Web Bundles.

Bug: 1082020
Change-Id: I231e8c85f740e3588c2ba4c518378cd26a36a79d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2275986
Reviewed-by: Hayato Ito <hayato@chromium.org>
Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#786147}