Consider intrinsic size for SVG as list-style-image

When using an SVG as list marker image, don't unconditionally set the
dimensions to a square with the dimensions of ascent / 2, but rather
determine a suitable size using intrinsic dimensions (and ratio).

This aligns behavior with Gecko and IE.

BUG=350734

Review URL: https://codereview.chromium.org/197203003

git-svn-id: svn://svn.chromium.org/blink/trunk@169053 bbb929c8-8fbe-4397-9dbb-9b2b20218538