Port policy::UploadJobImpl to network::SimpleURLLoader

This change ports UploadJobImpl from net::URLFetcher to SimpleURLLoader.

Since this request does not make use of the response a new method has
been added to SimpleURLLoader, DownloadToNull. This download mode still
reads the response body from the network but does not save it anywhere.
This is useful for requests which upload data and would otherwise have
to specify an arbitrary |max_body_size|. A maximum may still be set if
the caller wants to limit the amount of data transferred over the
network.

Bug: 773295
Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: Iee1fdc6f7406066ced8c91122e22cd51ddcb1c5f
Reviewed-on: https://chromium-review.googlesource.com/1161551
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581060}
8 files changed