Avoid duplicating the CSS property mapping for SVG pres. attrs.

The attribute (object) -> CSS property mapping is specified at
construction and stored, so rather than having to repeat it, just read
and use the stored value.

Also add a cssValue() helper method to SVGAnimatedPath.

Review-Url: https://codereview.chromium.org/2708923011
Cr-Commit-Position: refs/heads/master@{#452839}
11 files changed