platform2: Move UI classes from login_manager to libchromeos

Move the XServerRunner and ChromiumCommandBuilder classes
from login_manager into a ui/ directory in libchromeos.

BUG=chromium:377301
TEST=did a build and ran unit tests
CQ-DEPEND=I49a02a03708357903e386d48dac1c790f184b0de
CQ-DEPEND=I85a557fb42497c2e2c6c607b3df3bc0142a3c928

Change-Id: Id8c816f2388292ef1efb64985ee4e7321c394fde
Reviewed-on: https://chromium-review.googlesource.com/202592
Reviewed-by: Chris Masone <cmasone@chromium.org>
Commit-Queue: Daniel Erat <derat@chromium.org>
Tested-by: Daniel Erat <derat@chromium.org>
13 files changed