Add support for persistent notifications
Fixup showNotification
Use ServiceWorkerRegistration.pushRegistrationManager
Temporarily bypass hasPush check

Trunk builds of Chrome have no way to feature detect whether push is supported
until you get an instance of ServiceWorkerRegistration. This patch temporarily
disables the hasPush check to accomodate these builds.
Temporarily stop requiring no tabs to be open
Try to focus a tab when notification is clicked
Add "gcm_user_visible_only":true to manifest