Add cast shell wm_helper class

Add enable_cast_wayland_server flag to selectively enable exo server use.
Add a Cast-specific implementation of wm_helper.
This implementation contains the minimum required to make exo work on cast shell.

support EXO.

Bug: 896710
Test: Built on ChromeOS, is not utilized yet as cast does not yet
Change-Id: I0e38a415fbdecdbe7f116885ed9048834cbca1b5
Reviewed-on: https://chromium-review.googlesource.com/c/1294835
Commit-Queue: Shawn Gallea <sagallea@google.com>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605851}
6 files changed