exo: Run exo in mus+ash

Add WMHelperAsh and WMHelperMus. They are for accessing WM
related features in ash and mus+ash, and exo will create one
of those two helpers based on the WM environment.

Know issue: All methods of WMHelperMus are not implemented,
so the exo doesn't response to any events.

BUG=637914
TEST=manually with https://github.com/phuang/waylandcxx

Review-Url: https://codereview.chromium.org/2250863003
Cr-Commit-Position: refs/heads/master@{#412861}
11 files changed