Stats reported:
* Battery depletion percent over 5, 15 and 30 minute period.
* Battery discharge rate if battery was discharged for more than 30 minutes.
Reports are only generated after 30 minutes of uptime so as not to be affected by increased power draw at system startup.
In-progress stat collection is cancelled on sleep or when all renderers are closed.
BUG=418407
Review URL: https://codereview.chromium.org/560553005
Cr-Commit-Position: refs/heads/master@{#304412}
