Allow passing listening socket path on command line

Allow passing listening socket path on command line

When enabling externally-running applications, allow the
user to specify a path for the listening socket. This makes
development easier while still allowing for a default
to be used in production.

BUG=None
TEST=run mojo_shell with --enable-external-applications=path/to/socket, without the flag, and with the flag but passing no value.

Committed: https://crrev.com/774d386084e8cf996074ec1fa513e19a1cc1be62
Cr-Commit-Position: refs/heads/master@{#297247}

[Chris Masone, 2014-09-29 19:19:29]

cmasone@chromium.org changed reviewers:
+ davemoore@chromium.org

[Chris Masone, 2014-09-29 20:17:05]

PTAL

[DaveMoore, 2014-09-29 20:20:47]

lgtm

[Chris Masone, 2014-09-29 20:26:16]

The CQ bit was checked by cmasone@chromium.org

[commit-bot: I haz the power, 2014-09-29 20:27:42]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/599733007/1

[commit-bot: I haz the power, 2014-09-29 20:32:43]

Committed patchset #1 (id:1) as 4c7909794370733c259c324ebd4a876e1f7889a6

[commit-bot: I haz the power, 2014-09-29 20:33:29]

Patchset 1 (id:??) landed as
https://crrev.com/774d386084e8cf996074ec1fa513e19a1cc1be62
Cr-Commit-Position: refs/heads/master@{#297247}