Enable Battery Status API by default in Blink.

Set Battery Status API status to stable in blink. We have
implementations for all platforms supported by blink now,
i.e. on Android, Windows, Linux, ChromeOS and Mac.

BUG=122593
TEST=http://jsbin.com/battery-status-test

Review URL: https://codereview.chromium.org/451583003

git-svn-id: svn://svn.chromium.org/blink/trunk@180296 bbb929c8-8fbe-4397-9dbb-9b2b20218538