Use ScopedTaskEnvironment instead of MessageLoop in components unit tests.

components/sync/engine_impl/js_sync_manager_observer_unittest.cc

Index: components/sync/engine_impl/js_sync_manager_observer_unittest.cc
diff --git a/components/sync/engine_impl/js_sync_manager_observer_unittest.cc b/components/sync/engine_impl/js_sync_manager_observer_unittest.cc
index 5279ab38d8afa30d129e279d4d75cc6ecd5b6ad7..52774eeb5cae6ad7463a9a6269ce0c31815fc9ab 100644
--- a/components/sync/engine_impl/js_sync_manager_observer_unittest.cc
+++ b/components/sync/engine_impl/js_sync_manager_observer_unittest.cc
@@ -8,6 +8,7 @@
 
 #include "base/location.h"
 #include "base/run_loop.h"
+#include "base/test/scoped_task_environment.h"
 #include "base/values.h"
 #include "components/sync/base/model_type.h"
 #include "components/sync/engine/connection_status.h"
@@ -33,7 +34,7 @@ class JsSyncManagerObserverTest : public testing::Test {
  private:
   // This must be destroyed after the member variables below in order
   // for WeakHandles to be destroyed properly.
-  base::MessageLoop message_loop_;
+  base::test::ScopedTaskEnvironment scoped_task_environment_;
 
  protected:
   StrictMock<MockJsEventHandler> mock_js_event_handler_;