GitLab

This adds an optional "ProxyErrorClient" client to the NetworkContext which
receives notifications of errors in the PAC script, as well as notifications
of failed URL loads that may have been proxy related.

(Reland of d576f34a).

Bug: 876568,851609,879491
Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: I08987d658879913d94c9b13b6fdc1de2a32a6d54
Reviewed-on: https://chromium-review.googlesource.com/1200076
Commit-Queue: Eric Roman <eroman@chromium.org>
Reviewed-by: Will Harris <wfh@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#588259}