Use ScopedTaskEnvironment instead of MessageLoop in /device/gamepad

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=mattreynolds@chromium.org

Change-Id: I36fdabd895249be6a9c2c00952b90a2ba3b0855e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1648203
Auto-Submit: Carlos Caballero <carlscab@google.com>
Reviewed-by: Matt Reynolds <mattreynolds@chromium.org>
Commit-Queue: Matt Reynolds <mattreynolds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667272}

device/gamepad/BUILD.gn

@@ -164,6 +164,7 @@ static_library("test_helpers") {
   public_deps = [
     ":gamepad",
     "//base",
+    "//base/test:test_support",
     "//device/gamepad/public/cpp:shared_with_blink",
     "//third_party/blink/public:blink_headers",
   ]

device/gamepad/gamepad_test_helpers.h

@@ -8,9 +8,9 @@
 #include <memory>
 
 #include "base/macros.h"
-#include "base/message_loop/message_loop.h"
 #include "base/synchronization/lock.h"
 #include "base/synchronization/waitable_event.h"
+#include "base/test/scoped_task_environment.h"
 #include "device/gamepad/gamepad_data_fetcher.h"
 #include "device/gamepad/gamepad_service.h"
 #include "device/gamepad/gamepad_shared_buffer.h"
@@ -58,11 +58,9 @@ class GamepadTestHelper {
   GamepadTestHelper();
   virtual ~GamepadTestHelper();
 
-  base::MessageLoop& message_loop() { return message_loop_; }
-
  private:
   // This must be constructed before the system monitor.
-  base::MessageLoop message_loop_;
+  base::test::ScopedTaskEnvironment scoped_task_environment_;
 
   DISALLOW_COPY_AND_ASSIGN(GamepadTestHelper);
 };