For the switch kEnableTransparentVisuals to be effective it requires
a compositing manager to run. Indeed ui::ChooseVisualForWindow checks
for the presence of the Atom _NET_WM_CM_S0 before looking for a 32
bit depth compatible visual.

BUG=589509

R=dpranke@chromium.org, piman@chromium.org, sky@chromium.org, danakj@chromium.org

Review-Url: https://codereview.chromium.org/2008833002
Cr-Commit-Position: refs/heads/master@{#397073}