Bug 940842 - 1/3 - Add will-change CSS property - r=dbaron

The current spec draft is at:
http://tabatkins.github.io/specs/css-will-change/

This CSS property is disabled by default and can be enabled by toggling:
  layout.css.will-change.enabled
Bug 940842 - 2/3 - Implement will-change in layers-building code - r=mattwoodrow
Bug 940842 - 3/3 - Add a mochitest covering repaint heuristics with will-change - r=mattwoodrow