Enable site engagement in incognito.

chrome/browser/engagement/site_engagement_service_factory.h

Index: chrome/browser/engagement/site_engagement_service_factory.h
diff --git a/chrome/browser/engagement/site_engagement_service_factory.h b/chrome/browser/engagement/site_engagement_service_factory.h
index 9537618f99771f7fd2394d2e46f52a430e13d854..f16923febbde41dd738bee617ff70c5759958fe1 100644
--- a/chrome/browser/engagement/site_engagement_service_factory.h
+++ b/chrome/browser/engagement/site_engagement_service_factory.h
@@ -17,7 +17,6 @@ class SiteEngagementService;
 // the associated SiteEngagementService.
 //
 // The default factory behavior is suitable for this factory as:
-// * there should be no site engagement tracking in incognito
 // * the site engagement service should be created lazily
 // * the site engagement service is needed in tests.
 class SiteEngagementServiceFactory : public BrowserContextKeyedServiceFactory {
@@ -38,6 +37,8 @@ class SiteEngagementServiceFactory : public BrowserContextKeyedServiceFactory {
   // BrowserContextKeyedServiceFactory:
   KeyedService* BuildServiceInstanceFor(
       content::BrowserContext* profile) const override;
+  content::BrowserContext* GetBrowserContextToUse(
+      content::BrowserContext* context) const override;
 
   DISALLOW_COPY_AND_ASSIGN(SiteEngagementServiceFactory);
 };