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

chrome/browser/sync/user_event_service_factory.h

Index: chrome/browser/sync/user_event_service_factory.h
diff --git a/chrome/browser/sync/user_event_service_factory.h b/chrome/browser/sync/user_event_service_factory.h
index 0cc98061332951249c731b1dc5b4bf07fc6bcc89..26d2b444068cdb8a618901e7ebf71d61c4450813 100644
--- a/chrome/browser/sync/user_event_service_factory.h
+++ b/chrome/browser/sync/user_event_service_factory.h
@@ -31,6 +31,8 @@ class UserEventServiceFactory : public BrowserContextKeyedServiceFactory {
   // BrowserContextKeyedServiceFactory:
   KeyedService* BuildServiceInstanceFor(
       content::BrowserContext* context) const override;
+  content::BrowserContext* GetBrowserContextToUse(
+      content::BrowserContext* context) const override;
 
   DISALLOW_COPY_AND_ASSIGN(UserEventServiceFactory);
 };