Backport some tests in /components/web_resource to SingleThreadTaskEnvironment

These tests were previously migrated from single-threaded MessageLoop to
a multi-threaded TaskEnvironment (then named ScopedTaskEnvironment) as
part of crbug.com/891670.

//base OWNERS decided in retrospect that it was better to keep a
single-threaded option for TaskEnvironment and introduced
SingleThreadTaskEnvironment. This CL retrofits that decision for
/components/web_resource.

This CL is a no-op if it passes CQ.

This CL was uploaded by git cl split.

R=rsesek@chromium.org

Bug: 891670
Change-Id: I289ecffa9181d7090d1e052f942529a0d672b727
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786841
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693748}

components/web_resource/web_resource_service_unittest.cc

@@ -144,7 +144,7 @@ class WebResourceServiceTest : public testing::Test {
   void CallStartFetch() { test_web_resource_service_->StartFetch(); }
 
  private:
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   network::TestURLLoaderFactory test_url_loader_factory_;
   scoped_refptr<network::SharedURLLoaderFactory> test_shared_loader_factory_;
   std::unique_ptr<TestingPrefServiceSimple> local_state_;