Don't hard-code usage string in mojo_run.

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 shell_arguments
 from devtoolslib import shell_config
 
-_USAGE = ("mojo_run "
-         "[--args-for=<mojo-app>] "
-         "[--content-handlers=<handlers>] "
-         "[--enable-external-applications] "
-         "[--disable-cache] "
-         "[--enable-multiprocess] "
-         "[--wait-for-debugger] "
-         "[--sky <mojo-app>|<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.
+
+Important shell arguments include:
+  "--args-for=<mojo-app-url> <arguments>"
+  "--content-handlers=<handlers>"
+  "--disable-cache"
+  "--enable-multiprocess"
+  "--wait-for-debugger"
+  "<mojo-app-url>"
+  "<mojo-app-url> <arguments>"
+
+The value of <handlers> is a comma separated list like:
+text/html,mojo:html_viewer,application/javascript,mojo:js_content_handler
 """
 
-
 # Port on which the mojo:debugger http server will be available on the host
 # machine.
 _MOJO_DEBUGGER_PORT = 7777
 _DEFAULT_WM = 'https://core.mojoapps.io/kiosk_wm.mojo'
 
 _LEGACY_WM_URL = 'mojo:window_manager'
 _WM_URL = 'https://core.mojoapps.io/window_manager.mojo'
 _DEBUGGER_URL = 'https://core.mojoapps.io/debugger.mojo'
 
 
 def _configure_debugger(shell):
   """Configures debugger.mojo 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.forward_host_port_to_shell(_MOJO_DEBUGGER_PORT)
   return ['%s %d' % (_DEBUGGER_URL, _MOJO_DEBUGGER_PORT)]
 
 
 def main():
   logging.basicConfig()
 
-  parser = argparse.ArgumentParser(usage=_USAGE, description=_DESCRIPTION)
+  parser = argparse.ArgumentParser(
+      formatter_class=argparse.RawDescriptionHelpFormatter,
+      description=_DESCRIPTION)
   shell_config.add_shell_arguments(parser)
 
   parser.add_argument('--embed', type=str,
                       help='Url to be embedded in the window manager.')
   parser.add_argument('--window-manager', default=_DEFAULT_WM,
                       help='Window manager app to be mapped as '
                       'mojo:window_manager. By default it is ' +
                       _DEFAULT_WM)
   parser.add_argument('--debugger', action="store_true",
                       help='Run debugger.mojo along with the app.')
@@ skipping 26 matching lines @@
 
   if script_args.verbose:
     print "Shell arguments: " + str(shell_args)
 
   shell.run(shell_args)
   return 0
 
 
 if __name__ == "__main__":
   sys.exit(main())