Support running with mojo:debugger in devtools.

mojo/devtools/common/devtoolslib/shell_arguments.py

 # Copyright 2015 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.
 
 """Functions that configure the shell before it is run manipulating its argument
 list."""
 
 import urlparse
 
 _MAP_ORIGIN_PREFIX = '--map-origin='
 # When spinning up servers for local origins, we want to use predictable ports
 # so that caching works between subsequent runs with the same command line.
 _MAP_ORIGIN_BASE_PORT = 31338
 
+# Port on which the mojo:debugger http server will be available on the host
+# machine.
+_MOJO_DEBUGGER_PORT = 7777
+
 
 def _IsMapOrigin(arg):
   """Returns whether |arg| is a --map-origin argument."""
   return arg.startswith(_MAP_ORIGIN_PREFIX)
 
 
 def _Split(l, pred):
   positive = []
   negative = []
   for v in l:
@@ skipping 31 matching lines @@
   Returns:
     The updated argument list.
   """
   map_arguments, other_arguments = _Split(original_arguments, _IsMapOrigin)
   arguments = other_arguments
   next_port = _MAP_ORIGIN_BASE_PORT
   for mapping in sorted(map_arguments):
     arguments.append(_RewriteMapOriginParameter(shell, mapping, next_port))
     next_port += 1
   return arguments
+
+
+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 ['--args-for=mojo:debugger %d' % _MOJO_DEBUGGER_PORT,
+          'mojo:debugger']

[qsr, 2015/06/03 10:38:19]

If you plan to run mojo:debuffer, you can pass the argument by just returning:
['mojo:debugger %d' _MOJO_DEBUGGER_PORT]

[ppi, 2015/06/03 10:43:13]

Done.