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

chrome/browser/sync/user_event_service_factory.cc

Index: chrome/browser/sync/user_event_service_factory.cc
diff --git a/chrome/browser/sync/user_event_service_factory.cc b/chrome/browser/sync/user_event_service_factory.cc
index de359567b8bb9d3bd7ff437c875ee788802deb9f..cd00d510dde58ffb479cebdbb44294e7465a7390 100644
--- a/chrome/browser/sync/user_event_service_factory.cc
+++ b/chrome/browser/sync/user_event_service_factory.cc
@@ -15,7 +15,7 @@
 #include "components/keyed_service/content/browser_context_dependency_manager.h"
 #include "components/sync/base/model_type.h"
 #include "components/sync/base/report_unrecoverable_error.h"
-#include "components/sync/user_events/user_event_service.h"
+#include "components/sync/user_events/user_event_service_impl.h"
 #include "components/sync/user_events/user_event_sync_bridge.h"
 
 namespace browser_sync {
@@ -51,7 +51,7 @@ KeyedService* UserEventServiceFactory::BuildServiceInstanceFor(
                                               chrome::GetChannel()));
   auto bridge = base::MakeUnique<syncer::UserEventSyncBridge>(
       std::move(store_factory), std::move(processor_factory));
-  return new syncer::UserEventService(
+  return new syncer::UserEventServiceImpl(
       ProfileSyncServiceFactory::GetForProfile(profile), std::move(bridge));
 }