[Sync] Create UserEventService.

components/sync/BUILD.gn

Index: components/sync/BUILD.gn
diff --git a/components/sync/BUILD.gn b/components/sync/BUILD.gn
index a3e8b54023eba791fec27ec5c5fe4a36fccbb259..4b6f72d912ce85e186083d410bc089ac6fef8960 100644
--- a/components/sync/BUILD.gn
+++ b/components/sync/BUILD.gn
@@ -572,6 +572,8 @@ static_library("sync") {
     "syncable/write_transaction.h",
     "syncable/write_transaction_info.cc",
     "syncable/write_transaction_info.h",
+    "user_events/user_event_service.cc",
+    "user_events/user_event_service.h",
     "user_events/user_event_sync_bridge.cc",
     "user_events/user_event_sync_bridge.h",
   ]
@@ -587,6 +589,7 @@ static_library("sync") {
     "//base/third_party/dynamic_annotations",
     "//components/data_use_measurement/core",
     "//components/invalidation/public",
+    "//components/keyed_service/core",
     "//components/metrics",
     "//components/os_crypt",
     "//components/pref_registry",
@@ -942,6 +945,7 @@ 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_sync_bridge_unittest.cc",
   ]