BatteryStatus: fix chargingTime() to follow the specification.

If the battery status is unknown, the charging time should be
0 instead of Infinity. The reason being that in that case, the
battery status is reflected as a plugged and fully charged battery.

BUG=122593

Review URL: https://codereview.chromium.org/203693003

git-svn-id: svn://svn.chromium.org/blink/trunk@170338 bbb929c8-8fbe-4397-9dbb-9b2b20218538