SVGStringList update tests.

Update the SVGStringList tests to use a SVGStringList other than requiredFeatures,
as we need to remove requiredFeatures.
Fix SVGStringList-basics.
Make SVGStringList.js use requiredExtensions instead of requiredFeatures.

BUG=635420

Review-Url: https://codereview.chromium.org/2780443002
Cr-Commit-Position: refs/heads/master@{#460120}
20 files changed