Add support to Battery Status API in blink.

This patch adds basic Battery Manager to Navigator with default battery
status values and the test enables to test if those values are passed
correctly.

Intent-to-Implement link: https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/fzoG6Phr09k/dnAEzkgMA1YJ

BUG=122593

Review URL: https://codereview.chromium.org/182613002

git-svn-id: svn://svn.chromium.org/blink/trunk@168632 bbb929c8-8fbe-4397-9dbb-9b2b20218538