Simplify SVGAnimatedHref

Let SVGAnimatedHref hold the actual value for 'href' and just wrap the
object for 'xlink:href'. This means we can get rid of most of the
overrides.

BUG=584142

Review URL: https://codereview.chromium.org/1697593002

Cr-Commit-Position: refs/heads/master@{#375861}