Use ScopedTaskEnvironment instead of MessageLoop in /extensions

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=lazyboy@chromium.org

Change-Id: I235f4f603407ea113d949297e550d4c75dbf94ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1649580
Commit-Queue: Carlos Caballero <carlscab@google.com>
Auto-Submit: Carlos Caballero <carlscab@google.com>
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667176}

extensions/browser/preload_check_group_unittest.cc

@@ -5,7 +5,7 @@
 #include <memory>
 #include <vector>
 
-#include "base/message_loop/message_loop.h"
+#include "base/test/scoped_task_environment.h"
 #include "extensions/browser/preload_check_group.h"
 #include "extensions/browser/preload_check_test_util.h"
 #include "testing/gmock/include/gmock/gmock.h"
@@ -50,8 +50,8 @@ class PreloadCheckGroupTest : public testing::Test {
   std::unique_ptr<PreloadCheckGroup> check_group_;
 
  private:
-  // A message loop is required for the asynchronous tests.
-  base::MessageLoop message_loop_;
+  // Required for the asynchronous tests.
+  base::test::ScopedTaskEnvironment scoped_task_environment_;
 };
 
 // Tests multiple succeeding checks.