EXO: Enable wayland server in cast

Conditionally instantiate the Wayland server based on a BUILDFLAG in
Chromecast.

This change also splits out the WaylandWatcher from Ash code so that it
is shared between different builds.

Bug: 896710
Test: Compile cast_shell
Change-Id: I2d2e129add5f0bcf6f2368909c5c83b2ad00a9cc
Reviewed-on: https://chromium-review.googlesource.com/c/1329858
Commit-Queue: Daniel Nicoara <dnicoara@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Reviewed-by: Alex Sakhartchouk <alexst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613746}
12 files changed