Backport some tests in /components/subresource_filter 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/subresource_filter.

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

This CL was uploaded by git cl split.

R=pkalinnikov@chromium.org

Bug: 891670
Change-Id: I2b50b464bf35451ba478a29b25888b4c4ee97854
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786808
Auto-Submit: Gabriel Charette <gab@chromium.org>
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#696026}

components/subresource_filter/content/browser/async_document_subresource_filter_unittest.cc

@@ -77,7 +77,7 @@ class AsyncDocumentSubresourceFilterTest : public ::testing::Test {
   // Note: ADSF assumes a task runner is associated with the current thread.
   // Instantiate a MessageLoop on the current thread and use base::RunLoop to
   // handle the replies ADSF tasks generate.
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   scoped_refptr<base::TestSimpleTaskRunner> blocking_task_runner_ =
       new base::TestSimpleTaskRunner;