Backport some tests in /components/domain_reliability 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/domain_reliability.

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

This CL was uploaded by git cl split.

R=eroman@chromium.org

Bug: 891670
Change-Id: Ia403dd784ffbbee2f9ca96e39bd58af6072cbb52
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786856
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Eric Roman <eroman@chromium.org>
Commit-Queue: Eric Roman <eroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693876}

components/domain_reliability/uploader_unittest.cc

@@ -196,8 +196,8 @@ class DomainReliabilityUploaderTest : public testing::Test {
   }
 
  private:
-  base::test::TaskEnvironment task_environment_{
-      base::test::TaskEnvironment::MainThreadType::IO};
+  base::test::SingleThreadTaskEnvironment task_environment_{
+      base::test::SingleThreadTaskEnvironment::MainThreadType::IO};
   scoped_refptr<net::TestURLRequestContextGetter> url_request_context_getter_;
   UploadInterceptor* interceptor_;
   MockTime time_;