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This implements the 'local url' handling per:

  https://drafts.csswg.org/css-values/#local-urls

which is also referenced from:

  https://svgwg.org/svg2-draft/linking.html#linkRefAttrs

Most of the logic is handled by a new helper class named
SVGURLReferenceResolver, which keeps state, resolves the URL
and extracts the fragment identifier as needed.

BUG=470608

Review-Url: https://codereview.chromium.org/2174833002
Cr-Commit-Position: refs/heads/master@{#407299}