Updating MutableEvent initializers to match DOM spec (and current implementations)

Some references for these changes.

http://dev.w3.org/2006/webapi/DOM-Level-3-Events/html/DOM3-Events.html#interface-CustomEvent

http://dev.w3.org/2006/webapi/DOM-Level-3-Events/html/DOM3-Events.html#event-initializers

http://dev.w3.org/2006/webapi/DOM-Level-3-Events/html/DOM3-Events.html#idl-interface-UIEvent-initializers