Modernize mus_demo_external impl

This switches MusDemoExternal implementation to make use of
newly added WindowTreeClient::ConnectViaWindowTreeHostFactory API.

It allows us to remove some class parameters of WindowTreeDataExternal

BUG=666958

Change-Id: I12cb8e63d5bcd0c9484811b7cb09e0e546f866cb
Reviewed-on: https://chromium-review.googlesource.com/768987
Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/master@{#516359}
3 files changed