[CSS Typed OM] Implement CSSUnitValue::to for fixed length types

This applies when converting CSSUnitValues to other CSSUnitValues.

https://drafts.css-houdini.org/css-typed-om/#dom-cssnumericvalue-to

BUG=545318

Review-Url: https://codereview.chromium.org/2958233002
Cr-Commit-Position: refs/heads/master@{#485198}
4 files changed