devtools: new workflow for multiple simultaneous `mojo_run` runs.

devtools: new workflow for multiple simultaneous `mojo_run` runs.

This patch provides a new workflow for multiple simultaneous runs of
`mojo_run`. In the new workflow first instance of `mojo_run` is started
normally, while the subsequent ones get the `--reuse-servers` flag.

`--reuse-servers` skips spawning any dev servers, instead it assumes
that they are already running and only sets up port forwarding if
needed. This preserves caching and should work over adb_remote_setup.

The previous workaround switches: `--free-ports` and `--free-host-ports`
are removed.

Fixes https://github.com/domokit/devtools/issues/55 .

R=qsr@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/b80202520e08230ac7c114569ffc0840581b4e0c

[ppi, 2015-11-09 14:34:17]

Hi Ben,

ptal.

[ppi, 2015-11-09 16:11:28]

Added asserts as discussed offline, ptal.

[qsr, 2015-11-09 16:17:22]

lgtm

[ppi, 2015-11-09 16:18:33]

Committed patchset #3 (id:40001) manually as
b80202520e08230ac7c114569ffc0840581b4e0c (presubmit successful).