Always enable Oilpan for EventSource.

Source/core/page/EventSource.idl

 /*
  * Copyright (C) 2009 Ericsson AB. All rights reserved.
  * Copyright (C) 2010, 2011 Apple Inc. All Rights Reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
@@ skipping 18 matching lines @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 // https://html.spec.whatwg.org/multipage/comms.html#the-eventsource-interface
 
 [
     ActiveDOMObject,
     Constructor(DOMString url, optional EventSourceInit eventSourceInit),
     ConstructorCallWith=ExecutionContext,
     Exposed=(Window,Worker),
+    GarbageCollected,
     RaisesException=Constructor,
 ] interface EventSource : EventTarget {
     readonly attribute DOMString url;
     readonly attribute boolean withCredentials;
 
     // ready state
     const unsigned short CONNECTING = 0;
     const unsigned short OPEN = 1;
     const unsigned short CLOSED = 2;
     readonly attribute unsigned short readyState;
 
     // networking
     attribute EventHandler onopen;
     attribute EventHandler onmessage;
     attribute EventHandler onerror;
     void close();
 };