This was to support doing background: -webkit-canvas(#id) where you could then
get the canvas with document.getCSSCanvasContext(id) and draw into it which
is a poor form of custom drawing for elements. In Sky we'll have real custom
painting instead of just background images hacked in like this.
R=eseidel@chromium.org
Review URL: https://codereview.chromium.org/876913002