Add BUILD.gn rules to compile third_party/class-dump.

testing/iossim/redirect-stdout.sh

-#!/bin/sh
-# 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.
-#
-# This script executes the command given as the first argument, strips
-# cxx_destruct from stdout, and redirects the remaining stdout to the file given
-# as the second argument.
-#
-# Example: Write the text 'foo' to a file called out.txt:
-#   RedirectStdout.sh "echo foo" out.txt
-#
-# This script is invoked from iossim.gyp in order to redirect the output of
-# class-dump to a file (because gyp actions don't support redirecting output).
-# This script also removes all lines with cxx_destruct. Perhaps newer versions
-# of class-dump will fix this issue.  As of 3.5, 'cxx_destruct' still exists.
-
-if [ ${#} -ne 2 ] ; then
-  echo "usage: ${0} <command> <output file>"
-  exit 2
-fi
-
-echo "// Treat class-dump output as a system header." > $2
-echo "#pragma clang system_header" >> $2
-$1 | sed /cxx_destruct/d >> $2