sync: fix tiny bug with NotificationServiceSessionsRouter

On Chrome os managed mode, if we get a blocked navigation before session sync is set up we can't do anything with it.

BUG=98892

Review URL: https://codereview.chromium.org/132263003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244001 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/sync/sessions2/notification_service_sessions_router.cc

@@ -135,7 +135,7 @@ void NotificationServiceSessionsRouter::OnNavigationBlocked(
     content::WebContents* web_contents) {
   SyncedTabDelegate* tab =
       SyncedTabDelegate::ImplFromWebContents(web_contents);
-  if (!tab)
+  if (!tab || !handler_)
     return;
 
   DCHECK(tab->profile() == profile_);