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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.

Review URL: https://codereview.chromium.org/599733007

Cr-Commit-Position: refs/heads/master@{#297247}