Oilpan: move DOM string collection objects to the Oilpan heap.

Source/core/dom/DOMStringList.idl

 /*
  * Copyright (C) 2010 Google Inc. All Rights Reserved.
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 [
+    WillBeGarbageCollected,
 ] interface DOMStringList {
     readonly attribute unsigned long length;
     [TreatReturnedNullStringAs=Null] getter DOMString item([Default=Undefined] optional unsigned long index);
     [MeasureAs=DOMStringListContains] boolean contains([Default=Undefined] optional DOMString string);
 };