Findit identified the culprit r580911 with confidence 94.6% in the config "chromium.linux / Linux Tests (dbg)(1)(32)"
based on the flakiness trend:

https://findit-for-me.appspot.com/waterfall/flake?key=ag9zfmZpbmRpdC1mb3ItbWVyuQELEhdNYXN0ZXJGbGFrZUFuYWx5c2lzUm9vdCKCAWNocm9taXVtLmxpbnV4L0xpbnV4IFRlc3RzIChkYmcpKDEpKDMyKS81MTg4My9uZXR3b3JrX3NlcnZpY2VfZXh0ZW5zaW9uc19icm93c2VydGVzdHMvVjJWaVZtbGxkMEZRU1ZSbGMzUXVWR1Z6ZEZkbFlsSmxjWFZsYzNSQlVFaz0MCxITTWFzdGVyRmxha2VBbmFseXNpcxgBDA

If the culprit above is wrong, please file a bug using this link and hit submit:
https://bugs.chromium.org/p/chromium/issues/entry?status=Unconfirmed&labels=Pri-1,Test-Findit-Wrong&components=Tools%3ETest%3EFindit%3EFlakiness&summary=%5BFindit%5D%20Flake%20Analyzer%20-%20Wrong%20result%20for%20WebViewAPITest.TestWebRequestAPI&comment=Link%20to%20Analysis%3A%20https://findit-for-me.appspot.com/waterfall/flake?key=ag9zfmZpbmRpdC1mb3ItbWVyuQELEhdNYXN0ZXJGbGFrZUFuYWx5c2lzUm9vdCKCAWNocm9taXVtLmxpbnV4L0xpbnV4IFRlc3RzIChkYmcpKDEpKDMyKS81MTg4My9uZXR3b3JrX3NlcnZpY2VfZXh0ZW5zaW9uc19icm93c2VydGVzdHMvVjJWaVZtbGxkMEZRU1ZSbGMzUXVWR1Z6ZEZkbFlsSmxjWFZsYzNSQlVFaz0MCxITTWFzdGVyRmxha2VBbmFseXNpcxgBDA

Automatically posted by the findit-for-me app (https://goo.gl/Ot9f7N).

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/0b13d4396f5ad625ee44cd0d459857e2f9e97b83

commit 0b13d4396f5ad625ee44cd0d459857e2f9e97b83
Author: Clark DuVall <cduvall@chromium.org>
Date: Tue Aug 07 19:10:58 2018

Reland "Reland "Fix ExtensionWebRequestApiTest.WebRequestUnloadImmediately with network service""

This is a reland of 41b81861d982406ad5fdc69315d40e6b13d59d52

The original change was reverted for an old build that didn't have the fix in the reland.

Original change's description:
> Reland "Fix ExtensionWebRequestApiTest.WebRequestUnloadImmediately with network service"
>
> This is a reland of c9cbab072a2c44b00d21f6b2a19b2699f60f8e71
>
> request_completed_ was being set unnecessarily after RemoveRequest was called.
>
> Original change's description:
> > Fix ExtensionWebRequestApiTest.WebRequestUnloadImmediately with network service
> >
> > The error event was not being sent when the client was reset.
> >
> > Bug:  721414 
> > Cq-Include-Trybots: luci.chromium.try:linux_mojo
> > Change-Id: I951422061022d50644d8453900af14be27bb9eaf
> > Reviewed-on: https://chromium-review.googlesource.com/1150403
> > Commit-Queue: Clark DuVall <cduvall@chromium.org>
> > Reviewed-by: John Abd-El-Malek <jam@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#580911}
>
> TBR=jam@chromium.org
>
> Bug:  721414 
> Change-Id: I48769a4b86d2a18acbe5fbcedb9de3ed478e4c54
> Cq-Include-Trybots: luci.chromium.try:linux_mojo
> Reviewed-on: https://chromium-review.googlesource.com/1164209
> Reviewed-by: Clark DuVall <cduvall@chromium.org>
> Commit-Queue: Clark DuVall <cduvall@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#581058}

TBR=jam@chromium.org

Bug:  721414 ,  871601 
Change-Id: Ia42b661e48b61a6d1cffab453710160ac0f5b8d7
Cq-Include-Trybots: luci.chromium.try:linux_mojo
Reviewed-on: https://chromium-review.googlesource.com/1165503
Reviewed-by: Clark DuVall <cduvall@chromium.org>
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581299}
[modify] https://crrev.com/0b13d4396f5ad625ee44cd0d459857e2f9e97b83/extensions/browser/api/web_request/web_request_proxying_url_loader_factory.cc
[modify] https://crrev.com/0b13d4396f5ad625ee44cd0d459857e2f9e97b83/extensions/browser/api/web_request/web_request_proxying_url_loader_factory.h
[modify] https://crrev.com/0b13d4396f5ad625ee44cd0d459857e2f9e97b83/testing/buildbot/filters/mojo.fyi.network_browser_tests.filter