[Sync] Split fake ModelTypeService impl out of SMTP tests.

components/sync/BUILD.gn

Index: components/sync/BUILD.gn
diff --git a/components/sync/BUILD.gn b/components/sync/BUILD.gn
index b3de41140d89bf2a25feccb1ba53691a9a6c17a0..1e62aba7f9310278df139c713905a35da6cce8a7 100644
--- a/components/sync/BUILD.gn
+++ b/components/sync/BUILD.gn
@@ -716,6 +716,8 @@ static_library("test_support_sync_api") {
     "api/fake_syncable_service.h",
     "api/mock_model_type_store.cc",
     "api/mock_model_type_store.h",
+    "api/stub_model_type_service.cc",
+    "api/stub_model_type_service.h",
     "api/sync_change_processor_wrapper_for_test.cc",
     "api/sync_change_processor_wrapper_for_test.h",
     "api/sync_error_factory_mock.cc",
@@ -728,6 +730,7 @@ static_library("test_support_sync_api") {
     "//base",
     "//components/sync",
     "//testing/gmock",
+    "//testing/gtest",
   ]
 }