Adding ClearBlackList to the PreviewsBlackList and plumbing to UI

chrome/browser/previews/previews_service_factory.cc

Index: chrome/browser/previews/previews_service_factory.cc
diff --git a/chrome/browser/previews/previews_service_factory.cc b/chrome/browser/previews/previews_service_factory.cc
index e359487e28689e7a82b9c87dd6f1c8bf1d94e2a1..791aa5a190ffdd680c9b2c45728eba17419de207 100644
--- a/chrome/browser/previews/previews_service_factory.cc
+++ b/chrome/browser/previews/previews_service_factory.cc
@@ -11,21 +11,20 @@
 
 namespace {
 
 base::LazyInstance<PreviewsServiceFactory> g_previews_factory =
     LAZY_INSTANCE_INITIALIZER;
 
 }  // namespace
 
 // static
 PreviewsService* PreviewsServiceFactory::GetForProfile(Profile* profile) {
-  DCHECK_NE(profile->GetProfileType(), Profile::INCOGNITO_PROFILE);
   return static_cast<PreviewsService*>(
       GetInstance()->GetServiceForBrowserContext(profile, true));
 }
 
 // static
 PreviewsServiceFactory* PreviewsServiceFactory::GetInstance() {
   return g_previews_factory.Pointer();
 }
 
 PreviewsServiceFactory::PreviewsServiceFactory()