Implement DeviceLight

Initial implementation, behind a runtime flag.
This CL is meant for the blink side.
The android and Mac side code will be followed up in subsequent CLs.

Content/Chrome side CL with Android implementation -> https://codereview.chromium.org/152893002/

Spec: https://dvcs.w3.org/hg/dap/raw-file/default/light/Overview.html

Intent to Implement mail: https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/AAtN0xjI9Gc

BUG=336424

Review URL: https://codereview.chromium.org/143823004

git-svn-id: svn://svn.chromium.org/blink/trunk@173798 bbb929c8-8fbe-4397-9dbb-9b2b20218538