Support SVG fragment URLs in cross faded images

Extract url from CSSImageValue and pass it along to the SVGImage
wrapper. Constify CSSImageValue::url() in the process.

Also, fix typo and indentation in related test.

BUG=574172

Review URL: https://codereview.chromium.org/1585623003

Cr-Commit-Position: refs/heads/master@{#369156}
6 files changed