Use ScopedTaskEnvironment instead of MessageLoop

MessageLoop will go away, eventually.

ScopedTaskEnvironment will per default start a ThreadPool, which should
be fine in most of the cases. If you belive your test needs to make sure
that no ThreadPool runs let me know and I will update the patch.

BUG=891670
This CL was uploaded by git cl split.

R=jam@chromium.org

Change-Id: I5e7341427bf5ef241b73c3a82718c278d2401b2c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1635817Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Carlos Caballero <carlscab@google.com>
Auto-Submit: Carlos Caballero <carlscab@google.com>
Cr-Commit-Position: refs/heads/master@{#665081}

services/ws/gpu_host/gpu_host_unittest.cc

@@ -10,8 +10,8 @@
 #include "base/bind_helpers.h"
 #include "base/macros.h"
 #include "base/memory/weak_ptr.h"
-#include "base/message_loop/message_loop.h"
 #include "base/single_thread_task_runner.h"
+#include "base/test/scoped_task_environment.h"
 #include "components/discardable_memory/service/discardable_shared_memory_manager.h"
 #include "components/viz/host/gpu_client.h"
 #include "components/viz/service/gl/gpu_service_impl.h"
@@ -113,7 +113,7 @@ class GpuHostTest : public testing::Test {
   int GetChannelRequests();
 
  private:
-  base::MessageLoop message_loop_;
+  base::test::ScopedTaskEnvironment scoped_task_environment_;
 
   base::Thread io_thread_;
   TestGpuHostDelegate gpu_host_delegate_;