[Mojo Blob URLs] Handle downloads.

This changes the DownloadURL IPC to pass along a blob URL token, and
uses that to later construct a URLLoaderFactory for the blob URL.

Only works with the network service enabled, but at least fixes ordering
issues already present with the network service and downloading soon to
be revoked blob URLs.

Eventually this should let us remove the other blob specific codepaths
from (at least the network service branch of) the downloads code.

Design doc: https://docs.google.com/document/d/1DqVcTWE9Qb_3KpIRH2bFV-6hWEr8S92c4ppY67YL1KI/edit#heading=h.pvtybbkgf0vd

Bug: 800901, 808759
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: I5879fdc65f4843a1296a49496fc73ed6c3a04be1
Reviewed-on: https://chromium-review.googlesource.com/1026808
Commit-Queue: Marijn Kruisselbrink <[email protected]>
Reviewed-by: Min Qin <[email protected]>
Reviewed-by: Kinuko Yasuda <[email protected]>
Cr-Commit-Position: refs/heads/master@{#554418}
22 files changed