[CSS Typed OM] Update CSSPositionValue to match changed spec

- x and y are now settable
- Add missing exception handling in the constructor

Spec: https://drafts.css-houdini.org/css-typed-om/#positionvalue-objects

Bug: 545318
Change-Id: I73fef9ed11eccb5ed156d6279f69c373fe1ad7a3
Reviewed-on: https://chromium-review.googlesource.com/554654
Commit-Queue: meade_UTC10 <meade@chromium.org>
Reviewed-by: Eric Willigers <ericwilligers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487747}
8 files changed