Allow to pass additional shell arguments in `mojo_test`.

mojo/devtools/common/mojo_run

 #!/usr/bin/env python
 # Copyright 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.
 
 import argparse
 import logging
 import sys
 
 from devtoolslib import paths
 from devtoolslib import shell_arguments
 
-USAGE = ("mojo_run "
+_USAGE = ("mojo_run "
          "[--args-for=<mojo-app>] "
          "[--content-handlers=<handlers>] "
          "[--enable-external-applications] "
          "[--disable-cache] "
          "[--enable-multiprocess] "
          "[<mojo-app>] "
          """
 
 A <mojo-app> is a Mojo URL or a Mojo URL and arguments within quotes.
 Example: mojo_run "mojo:js_standalone test.js".
 <url-lib-path> is searched for shared libraries named by mojo URLs.
 The value of <handlers> is a comma separated list like:
 text/html,mojo:html_viewer,application/javascript,mojo:js_content_handler
 """)
 
+_DESCRIPTION = """Runner for Mojo applications.
+
+Any arguments not recognized by the script will be passed on as shell arguments.
+"""
+
 
 # Port on which the mojo:debugger http server will be available on the host
 # machine.
 _MOJO_DEBUGGER_PORT = 7777
 
 _DEFAULT_WINDOW_MANAGER = "mojo:kiosk_wm"
 
 
 def _ConfigureDebugger(shell):
   """Configures mojo:debugger to run and sets up port forwarding for its http
   server if the shell is running on a device.
 
   Returns:
     Arguments that need to be appended to the shell argument list in order to
     run with the debugger.
   """
   shell.ForwardHostPortToShell(_MOJO_DEBUGGER_PORT)
   return ['mojo:debugger %d' % _MOJO_DEBUGGER_PORT]
 
 
 def main():
   logging.basicConfig()
 
-  parser = argparse.ArgumentParser(usage=USAGE)
+  parser = argparse.ArgumentParser(usage=_USAGE, description=_DESCRIPTION)
 
   # Arguments allowing to indicate the configuration we are targeting when
   # running within a Chromium-like checkout. These will go away once we have
   # devtools config files, see https://github.com/domokit/devtools/issues/28.
   chromium_config_group = parser.add_argument_group('Chromium configuration',
       'These arguments allow to infer paths to tools and build results '
       'when running withing a Chromium-like checkout')
   debug_group = chromium_config_group.add_mutually_exclusive_group()
   debug_group.add_argument('--debug', help='Debug build (default)',
                            default=True, action='store_true')
@@ skipping 63 matching lines @@
 
   if script_args.verbose:
     print "Shell arguments: " + str(shell_args)
 
   shell.Run(shell_args)
   return 0
 
 
 if __name__ == "__main__":
   sys.exit(main())