[css-properties-values-api] Reify values from attributeStyleMap.get.

When retrieving values via attributeStyleMap.get/getAll, produce the
correct CSSStyleValue subclass according to the registration of the custom
property.

R=futhark@chromium.org

Bug: 641877
Change-Id: If564fb039506a22162ad2521479fcb1c3de9336c
Reviewed-on: https://chromium-review.googlesource.com/1172291
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Anders Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582569}
4 files changed