Introduce support for ExtendedAttributeIdentList in the IDL parser.

tools/idl_parser/test_parser/extattr_ppapi.idl

 /* Copyright 2013 The Chromium Authors. All rights reserved.
    Use of this source code is governed by a BSD-style license that can be
   found in the LICENSE file. */
 
 /* Test ExtendedAttribute productions
 
 Run with --test to generate an AST and verify that all comments accurately
 reflect the state of the Nodes.
 
 BUILD Type(Name)
@@ skipping 71 matching lines @@
 
 [foo=true] interface Foo {};
 
 /* TREE
  *Interface(Foo)
  *  ExtAttributes()
  *    ExtAttribute(foo)
  */
 
 [foo=1.2e-3] interface Foo {};
+
+/* TREE
+ *Interface(Foo)
+ *  ExtAttributes()
+ *    ExtAttribute(foo)
+ */
+
+[foo=(bar, baz)] interface Foo {};