Backport some tests in /chrome/browser/chromeos 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
/chrome/browser/chromeos.

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

This CL was uploaded by git cl split.

R=jamescook@chromium.org

Bug: 891670
Change-Id: I32b1cc16d0b36bdb79c3c7668aca9c90cbf0a4ed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1787140
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693873}

chrome/browser/chromeos/mobile/mobile_activator_unittest.cc

@@ -140,7 +140,7 @@ class MobileActivatorTest : public testing::Test {
     cellular_network_.connection_state_ = state;
   }
 
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   NetworkState cellular_network_;
   TestMobileActivator mobile_activator_;
 

chrome/browser/chromeos/net/network_throttling_observer_unittest.cc

@@ -50,7 +50,7 @@ class NetworkThrottlingObserverTest : public ::testing::Test {
   }
 
  private:
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   std::unique_ptr<NetworkStateHandler> network_state_handler_;
   std::unique_ptr<TestingPrefServiceSimple> local_state_;
   std::unique_ptr<NetworkThrottlingObserver> observer_;