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Add a flag for lucid sleep so that apps that wish to start working with it can
do so while it's still under development.  Also to help with testing when the
lid is closed, add a class that will flash the light bar when the system is in dark
resume.

BUG=397328
Signed-off-by: Chirantan Ekbote <chirantan@chromium.org>

Review URL: https://codereview.chromium.org/566993002

Cr-Commit-Position: refs/heads/master@{#294972}