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This reverts commit d576f34a.

Reason for revert: Likely culprit of https://crbug.com/879491.

Original change's description:
> Make chrome.proxy.onProxyError extension API work under the Network Service.
> 
> 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.
> 
> Bug: 876568,851609
> Cq-Include-Trybots: luci.chromium.try:linux_mojo
> Change-Id: I645ef91875135fcc81a7bd113442a0b65c661e65
> Reviewed-on: https://chromium-review.googlesource.com/1192123
> Reviewed-by: Matt Menke <mmenke@chromium.org>
> Reviewed-by: Will Harris <wfh@chromium.org>
> Commit-Queue: Eric Roman <eroman@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#587812}

TBR=eroman@chromium.org,mmenke@chromium.org,wfh@chromium.org

Change-Id: I286c7f768b04b6e47f1836abc459a33c7b394498
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 876568, 851609
Cq-Include-Trybots: luci.chromium.try:linux_mojo
Reviewed-on: https://chromium-review.googlesource.com/1199043Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587966}