[Sync] Split UserEventService into interface and impl, add fake impl, add unit tests.

components/sync/BUILD.gn

Index: components/sync/BUILD.gn
diff --git a/components/sync/BUILD.gn b/components/sync/BUILD.gn
index 8fa00f334f7072df28322a32b0d2d14f2546fe05..dd5217ca03e7c81eeafb79dbb9b052319b898e66 100644
--- a/components/sync/BUILD.gn
+++ b/components/sync/BUILD.gn
@@ -572,8 +572,12 @@ static_library("sync") {
     "syncable/write_transaction.h",
     "syncable/write_transaction_info.cc",
     "syncable/write_transaction_info.h",
+    "user_events/fake_user_event_service.cc",
+    "user_events/fake_user_event_service.h",
     "user_events/user_event_service.cc",
     "user_events/user_event_service.h",
+    "user_events/user_event_service_impl.cc",
+    "user_events/user_event_service_impl.h",
     "user_events/user_event_sync_bridge.cc",
     "user_events/user_event_sync_bridge.h",
   ]
@@ -945,7 +949,7 @@ source_set("unit_tests") {
     "syncable/syncable_enum_conversions_unittest.cc",
     "syncable/syncable_id_unittest.cc",
     "syncable/syncable_unittest.cc",
-    "user_events/user_event_service_unittest.cc",
+    "user_events/user_event_service_impl_unittest.cc",
     "user_events/user_event_sync_bridge_unittest.cc",
   ]