Backport some tests in /google_apis/gcm 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
/google_apis/gcm.

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

This CL was uploaded by git cl split.

R=peter@chromium.org

Bug: 891670
Change-Id: Ic359afa58bb711c2b5e681a3417fe020173ec740
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1787058
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Commit-Queue: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694183}

google_apis/gcm/engine/mcs_client_unittest.cc

@@ -173,7 +173,7 @@ class MCSClientTest : public testing::Test {
   base::SimpleTestClock clock_;
 
   base::ScopedTempDir temp_directory_;
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   std::unique_ptr<base::RunLoop> run_loop_;
   std::unique_ptr<GCMStore> gcm_store_;