Add `mojo_run --free-ports` to run servers on system-allocated ports.

mojo/devtools/common/devtoolslib/shell_config.py

Index: mojo/devtools/common/devtoolslib/shell_config.py
diff --git a/mojo/devtools/common/devtoolslib/shell_config.py b/mojo/devtools/common/devtoolslib/shell_config.py
index b3faa05ba773e99d7b6fa16aa267d77632c40a3c..79a14c2ed72453ada6fb5a53e5d665976bebd289 100644
--- a/mojo/devtools/common/devtoolslib/shell_config.py
+++ b/mojo/devtools/common/devtoolslib/shell_config.py
@@ -29,6 +29,7 @@ class ShellConfig(object):
     self.map_url_list = []
     self.map_origin_list = []
     self.dev_servers = []
+    self.free_ports = False
     self.content_handlers = dict()
     self.verbose = None
 
@@ -69,6 +70,10 @@ def add_shell_arguments(parser):
   parser.add_argument('--map-origin', action='append',
                       help='Define a mapping for a url origin in the format '
                       '<origin>=<url-or-local-file-path>')
+  parser.add_argument('--free-ports', action='store_true',
+                      help='Use system-allocated ports when spawning local '
+                      'servers. This defeats caching and thus hurts '
+                      'performance.')
   parser.add_argument('-v', '--verbose', action="store_true",
                       help="Increase output verbosity")
 
@@ -80,7 +85,9 @@ def add_shell_arguments(parser):
                              'additional logcat tags to display.')
   android_group.add_argument('--free-host-ports', action='store_true',
                              help='Use system-allocated ports on the host when '
-                             'spawning local servers.')
+                             'spawning local servers. This still forwards to '
+                             'fixed ports on the device, so that caching '
+                             'works.')
 
   desktop_group = parser.add_argument_group('Desktop-only',
       'These arguments apply only when running on desktop.')
@@ -153,6 +160,7 @@ def get_shell_config(script_args):
   shell_config.origin = script_args.origin
   shell_config.map_url_list = script_args.map_url
   shell_config.map_origin_list = script_args.map_origin
+  shell_config.free_ports = script_args.free_ports
   shell_config.verbose = script_args.verbose
 
   # Android-only.