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

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

This makes mojo devtools use system-allocated ports for all dev servers
that normally are run on fixed ports to facilitate caching (ie. dev
servers spawned for local --origin, --map-origin and --map-url).

Note that this does not affect dev servers with explicitly assigned
fixed ports in a mojoconfig file.

Fixes #477.

R=qsr@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/6f424d79b6c6a87827fcd5a119d7c87c96476bc2

[ppi, 2015-10-29 17:29:56]

ppi@chromium.org changed reviewers:
+ qsr@chromium.org

[ppi, 2015-10-29 17:29:57]

Hi Ben,

ptal.

[qsr, 2015-11-02 10:53:16]

LGTM with nits.

https://codereview.chromium.org/1419333005/diff/1/mojo/devtools/common/devtoo...
File mojo/devtools/common/devtoolslib/shell_arguments.py (right):

https://codereview.chromium.org/1419333005/diff/1/mojo/devtools/common/devtoo...
mojo/devtools/common/devtoolslib/shell_arguments.py:81:
<origin>=<url-or-local-path>.
Can you comment on free_ports and free_hosts_ports?

[ppi, 2015-11-02 13:04:18]

https://codereview.chromium.org/1419333005/diff/1/mojo/devtools/common/devtoo...
File mojo/devtools/common/devtoolslib/shell_arguments.py (right):

https://codereview.chromium.org/1419333005/diff/1/mojo/devtools/common/devtoo...
mojo/devtools/common/devtoolslib/shell_arguments.py:81:
<origin>=<url-or-local-path>.
On 2015/11/02 10:53:16, qsr wrote:
> Can you comment on free_ports and free_hosts_ports?

Done.

[ppi, 2015-11-02 13:14:08]

Committed patchset #2 (id:20001) manually as
6f424d79b6c6a87827fcd5a119d7c87c96476bc2 (presubmit successful).