Allow leading underscore on identifier names in IDL parser

tools/json_schema_compiler/test/idl_reserved_words.idl

+// Copyright (c) 2012 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.
+
+namespace reserved_words {
+
+  enum Foo { _float, _DOMString };
+
+  enum _enum {
+    _callback,
+    _namespace
+  };
+
+  dictionary _dictionary {
+    long _long;
+  };
+
+  dictionary MyType {
+    DOMString _interface;
+  };
+
+  interface Functions {
+    static void _static(Foo foo, _enum e);
+  };
+};