Use ScopedTaskEnvironment instead of MessageLoop in components unit tests.

components/sync/engine_impl/sync_manager_impl_unittest.cc

Index: components/sync/engine_impl/sync_manager_impl_unittest.cc
diff --git a/components/sync/engine_impl/sync_manager_impl_unittest.cc b/components/sync/engine_impl/sync_manager_impl_unittest.cc
index d7f0c79cdcc96814f09f5bdfe12c157a9938b6cd..96c6c1eeb7a1831a3b5126295e4392c496533124 100644
--- a/components/sync/engine_impl/sync_manager_impl_unittest.cc
+++ b/components/sync/engine_impl/sync_manager_impl_unittest.cc
@@ -18,6 +18,7 @@
 #include "base/strings/stringprintf.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/test/scoped_feature_list.h"
+#include "base/test/scoped_task_environment.h"
 #include "base/test/values_test_util.h"
 #include "base/values.h"
 #include "components/sync/base/attachment_id_proto.h"
@@ -255,7 +256,7 @@ class SyncApiTest : public testing::Test {
   PassphraseType GetPassphraseType(BaseTransaction* trans);
 
  private:
-  base::MessageLoop message_loop_;
+  base::test::ScopedTaskEnvironment scoped_task_environment_;
   TestUserShare test_user_share_;
 };
 
@@ -1235,7 +1236,7 @@ class SyncManagerTest : public testing::Test,
 
  private:
   // Needed by |sync_manager_|.
-  base::MessageLoop message_loop_;
+  base::test::ScopedTaskEnvironment scoped_task_environment_;
   // Needed by |sync_manager_|.
   base::ScopedTempDir temp_dir_;
   // Sync Id's for the roots of the enabled datatypes.