Remove will-change: contents GPU rasterization hint

We've decided not to ship this configuration. The implementation also doesn't
work because it incorrectly changes the paint order of elements with
will-change: contents by make them into a NormalLayer.

R=esprehn@chromium.org
BUG=370604

Review URL: https://codereview.chromium.org/270383002

git-svn-id: svn://svn.chromium.org/blink/trunk@173557 bbb929c8-8fbe-4397-9dbb-9b2b20218538