Use ScopedTaskEnvironment instead of MessageLoop in components unit tests.

components/sync/engine_impl/js_mutation_event_observer_unittest.cc

Index: components/sync/engine_impl/js_mutation_event_observer_unittest.cc
diff --git a/components/sync/engine_impl/js_mutation_event_observer_unittest.cc b/components/sync/engine_impl/js_mutation_event_observer_unittest.cc
index a6dfe74ac3519bccc31e11c8a9e2a03dcf2c4feb..1a8da574aef4618f112f6e3aaa5ee36d6e13644a 100644
--- a/components/sync/engine_impl/js_mutation_event_observer_unittest.cc
+++ b/components/sync/engine_impl/js_mutation_event_observer_unittest.cc
@@ -5,6 +5,7 @@
 #include "components/sync/engine_impl/js_mutation_event_observer.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/js/js_event_details.h"
@@ -27,7 +28,7 @@ class JsMutationEventObserverTest : 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_;