Add a watchlist for device_light in Blink.

Add a watchlist for the device_light module and its layout tests
in Blink.

BUG=

Review URL: https://codereview.chromium.org/341273005

git-svn-id: svn://svn.chromium.org/blink/trunk@176660 bbb929c8-8fbe-4397-9dbb-9b2b20218538