StockMenu was creating a container for the sole purpose of applying style to
PopupMenu. Now we just use a StyleNode.

Also, I've reverted the change to make box-sizing default to border-box. It
turns out that CL wasn't effective because we didn't use the initialBoxSizing
function to initialize box sizing. I've made us use initialBoxSizing but switch
the default back to content-box because actually using border-box breaks a
bunch of stuff.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1024083003