Set will-change while animating opacity/transform/filter

[dstoc]

The polyfill should set the will-change property while animating these properties.

See PolymerElements/neon-animation#119

[samuelli]

Will affect Safari & Firefox before they ship: http://caniuse.com/#feat=will-change

[Martin-Pitt]

Keep in mind that will-change is intended to be set before rather than while animating properties to warn the browser for optimisations.

Give it sufficient time to work. This property is intended as a method for authors to let the user-agent know about properties that are likely to change ahead of time. […]