Remove redundant [WillBeGarbageCollected] uses on derived EventTargets.

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 12 matching lines @@
  * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
  * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
  * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 [
-    WillBeGarbageCollected,
     ActiveDOMObject,
     Constructor(DOMString url, optional EventSourceInit eventSourceInit),
     ConstructorCallWith=ExecutionContext,
     Exposed=(Window,Worker),
     RaisesException=Constructor,
 ] interface EventSource : EventTarget {
 
     [DeprecateAs=EventSourceURL] readonly attribute DOMString URL; // Lowercased .url is the one in the spec, but leaving .URL for compatibility reasons.
     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();
 };