GitLab

(Partly as a preparation to add more logic for htxg-specific prefetch
plumbing)

Introduce prefetch-specific URLLoader which does:
- Basically just forwards the request to the underlying network loader
- Doesn't forward back the data to the renderer process, which used to
  be just discarded in the renderer process anyways.

This is hooked as follows:
- When NetworkService is enabled LinkLoader gets PrefetchURLLoaderFactory
  via PrefetchURLLoaderService and sets it as ResourceLoaderOptions.
  url_loader_factory.
- When NetworkService is NOT enabled this is directly hooked from the
  ResourceMessageFilter.

In both cases the URLLoaderFactory is owned by StoragePartition,
partially because it makes various plumbing (including tests) easier.

Optionally we can make it live longer than the client process, though
it is not currently implemented. (So no behavior change is expected
with this one)

Bug: 803776
Change-Id: Ia4b33048a7c6d27a80044d99b62a1cc5faad5681
Reviewed-on: https://chromium-review.googlesource.com/912051
Commit-Queue: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537605}