Bug 979084 - Enlarge displayport along one axis when the other is clipped. r=ajones

If a generated displayport expands beyond the scrollable rect on one axis,
redistribute that extra area to the other axis. Enabled by pref
apz.enlarge_displayport_when_clipped.
Bug 979084 - Adjust the displayport when one axis is clipped by default on b2g. r=ajones

Change the default displayport settings to redistribute wasted displayport area
from one axis to the other and adjust displayport size accordingly on b2g.