Remove -webkit-aspect-ratio.

This property is parsed and stored on RenderStyle, but it's not
used anywhere, i.e. it has no effect.

If we later decide that we want this after all, we can re-implement
it as 'aspect-ratio' behind a flag.

Review URL: https://codereview.chromium.org/647723002

git-svn-id: svn://svn.chromium.org/blink/trunk@183652 bbb929c8-8fbe-4397-9dbb-9b2b20218538