mus: Unset the cursor client during teardown.

DesktopWindowTreeHostMus owns the cursor client. So it is destroyed before
the ~WindowTreeHostMus is called. But the WindowTreeHostMus teardown can
try to access the cursor client. So it is necessary to unset the cursor
client on the root-window.

BUG=676184

Review-Url: https://codereview.chromium.org/2596723002
Cr-Commit-Position: refs/heads/master@{#440283}
2 files changed