Assigning to you for now John. Let me know if you need any more context/help. I think solving this should also help enable -ExtensionWebRequestApiTest.WebRequestURLFetcherInterception. 

Thanks.

I'm not sure it's quite this simple though - I don't think we want WebRequest listeners to see arbitrary requests made by the browser process on the profile's behalf.

It seems to me that features which need their requests to be visible to WebRequest should get their URLLoaderFactory elsewhere, or at least should have to explicitly opt-in to allowing interception so that StoragePartitionImpl can know it's OK to call ContentBrowserClient::WillCreateURLLoaderFactory (and thus possibly have proxies injected) before returning.

>> I'm not sure it's quite this simple though - I don't think we want WebRequest listeners to see arbitrary requests made by the browser process on the profile's behalf.

Isn't that the current behavior though? Unless the request is made from the system's request context.

Huh, I guess you're right. We just filter sensitive hosts. OK, SGTM

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/428b1d6eaee89f1f9caef8ee95aea358474a0448

commit 428b1d6eaee89f1f9caef8ee95aea358474a0448
Author: Ken Rockot <rockot@chromium.org>
Date: Wed Jun 06 17:12:21 2018

Proxy browser requests through WebRequest API

This enables browser-initiated requests to be proxied through the
WebRequest API when the Network Service is enabled. Note that this
only applies to requests made on via a profile's context, not the
system context.

This mirrors the behavior in the non-Network Service path today.

Bug:  841002 
Test: see buildbot filter changes
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: I1830959e1ba089974c546d933b4a6b51fb038779
Reviewed-on: https://chromium-review.googlesource.com/1083952
Commit-Queue: Ken Rockot <rockot@chromium.org>
Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#564937}
[modify] https://crrev.com/428b1d6eaee89f1f9caef8ee95aea358474a0448/chrome/browser/chrome_content_browser_client.cc
[modify] https://crrev.com/428b1d6eaee89f1f9caef8ee95aea358474a0448/chrome/browser/chrome_content_browser_client.h
[modify] https://crrev.com/428b1d6eaee89f1f9caef8ee95aea358474a0448/content/browser/frame_host/render_frame_host_impl.cc
[modify] https://crrev.com/428b1d6eaee89f1f9caef8ee95aea358474a0448/content/browser/loader/navigation_url_loader_impl.cc
[modify] https://crrev.com/428b1d6eaee89f1f9caef8ee95aea358474a0448/content/browser/storage_partition_impl.cc
[modify] https://crrev.com/428b1d6eaee89f1f9caef8ee95aea358474a0448/content/public/browser/content_browser_client.cc
[modify] https://crrev.com/428b1d6eaee89f1f9caef8ee95aea358474a0448/content/public/browser/content_browser_client.h
[modify] https://crrev.com/428b1d6eaee89f1f9caef8ee95aea358474a0448/extensions/browser/api/web_request/web_request_api.cc
[modify] https://crrev.com/428b1d6eaee89f1f9caef8ee95aea358474a0448/extensions/browser/api/web_request/web_request_proxying_url_loader_factory.cc
[modify] https://crrev.com/428b1d6eaee89f1f9caef8ee95aea358474a0448/extensions/browser/api/web_request/web_request_proxying_url_loader_factory.h
[modify] https://crrev.com/428b1d6eaee89f1f9caef8ee95aea358474a0448/extensions/shell/browser/shell_content_browser_client.cc
[modify] https://crrev.com/428b1d6eaee89f1f9caef8ee95aea358474a0448/extensions/shell/browser/shell_content_browser_client.h
[modify] https://crrev.com/428b1d6eaee89f1f9caef8ee95aea358474a0448/testing/buildbot/filters/mojo.fyi.network_browser_tests.filter