[Sensors] Adding chrome flag to enable Generic Sensor based APIs

This change adds a flag to enable family of sensor APIs which are based on Generic Sensor API including Ambient Light Sensor API.
So far it is only available on Android and Mac OS, should become available on other platforms as soon as platform-specific implementation lands.

Generic Sensor API: https://w3c.github.io/sensors/
Ambient Light Sensor API: http://w3c.github.io/ambient-light/

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

BUG=606766
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_site_isolation

Review-Url: https://codereview.chromium.org/2356193002
Cr-Commit-Position: refs/heads/master@{#424703}
11 files changed