Update task scheduler using service workers extensibility

First of all I'm bit new the concept and I plead for your patience with review. 

This merge request :
 - Updates task scheduler API spec using service workers extention API.
 - Proposing to remove hasPendingMessages() API from the task scheduler (I might be totally missing usecase here) as this feels redundant. Either there will be no pending messages as service worker will always get notified of the task fired, or one can use getPendingTasks to know if any tasks are active and pending system message. 

/cc @jungkees @anssiko 
Shouldnt we avoid saying "system message" - I have no idea what that is and isnt that what moz used for Firefox OS and in the original spec. This needs rewording which fits with the service worker.