servo: Merge #13475 - Disable EventSource until it is fully implemented (from servo:disable-eventsource); r=Manishearth

This also causes a number of tests that currently time out to fail quickly.

Source-Repo: https://github.com/servo/servo
Source-Revision: 0ee35a3b34a8b314ec2503354c385f5d58d70518

servo/components/script/dom/webidls/EventSource.webidl

@@ -7,7 +7,8 @@
  */
 
 [Constructor(DOMString url, optional EventSourceInit eventSourceInitDict),
- Exposed=(Window,Worker)]
+ Exposed=(Window,Worker),
+ Pref="dom.eventsource.enabled"]
 interface EventSource : EventTarget {
   readonly attribute DOMString url;
   readonly attribute boolean withCredentials;