Fix flaky ExtensionWebRequestApiTest.WebRequestTestOSDD with network service

This test uses an OSDD URL that gives a 404, so allow http errors for
the SimpleURLLoader. It looks like this could flake if there was a delay
between OnReceiveResponse and OnComplete.

Bug: 870588
Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: I101894bba7c31fcc7a686ab79b6e6bb936c4c26c
Reviewed-on: https://chromium-review.googlesource.com/1187667Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585894}

components/search_engines/template_url_fetcher.cc

@@ -115,6 +115,7 @@ TemplateURLFetcher::RequestDelegate::RequestDelegate(
                                  net::LOAD_DO_NOT_SEND_AUTH_DATA;
   simple_url_loader_ = network::SimpleURLLoader::Create(
       std::move(resource_request), kTrafficAnnotation);
+  simple_url_loader_->SetAllowHttpErrorResults(true);
   simple_url_loader_->DownloadToString(
       url_loader_factory,
       base::BindOnce(

testing/buildbot/filters/mojo.fyi.network_browser_tests.filter

@@ -108,10 +108,6 @@
 # the network service.
 -ExtensionWebRequestApiTest.WebRequestBlockingSetCookieHeader
 
-# https://crbug.com/870588
-# Flaky on Win with network service.
--ExtensionWebRequestApiTest.WebRequestTestOSDD
-
 # https://crbug.com/809583
 # Switch Predictor from using ResourceThrottle and ResourceDispatcherHost to
 # using WebContentsObserver and NavigationHandle.