Migrate dartium_tools from chrome branch to dart repo

tools/dartium/get_chromium_build.py

 #!/usr/bin/env python
 #
 # Copyright (c) 2014 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.
 
 """Gets a Chromium archived build, and unpacks it
    into a target directory.
 
   Use -r option to specify the revison number
   Use -t option to specify the directory to unzip the build into.
 
 Usage:
-  $ get_chromium_build.py -r <revision> -t <target>
+  $ /path/to/get_chromium_build.py -r <revision> -t <target>
 """
 
 import logging
 import optparse
 import os
 import platform
 import shutil
 import subprocess
 import sys
 import time
@@ skipping 137 matching lines @@
 
 def main(argv):
   logging.getLogger().setLevel(logging.DEBUG)
   options = ParseOptions(argv)
   b = BuildUpdater(options)
   b.DownloadAndUpdateBuild()
   logging.info('Successfully got archived Chromium build.')
 
 if __name__ == '__main__':
   sys.exit(main(sys.argv))