Adds ash::MaterialDesignController and a run-time flag

Adds "ash-enable-md" and "ash-disable-md" flags to enable / disable
Material Design in Chrome OS system UI.
ash::MaterialDesignController::IsMaterial() can be used to query the
mode at run-time.

BUG=605644

Review-Url: https://codereview.chromium.org/1921133002
Cr-Commit-Position: refs/heads/master@{#390260}
13 files changed