Return scoped_refptr<CSSVariableData> from ValueFromCustomProperty.

In the near future, it will be necessary for ValueForCustomProperty
to return a CSSVariableData which may not be stored elsewhere. This
is because we need to do a "non-destructive" pre-resolve of registered
custom properties that are used before high priority properties are
applied.

R=futhark@chromium.org

Bug: 641877
Change-Id: Idb1ab4ba7aae77187a4fd86877a0246e56fc82a4
Reviewed-on: https://chromium-review.googlesource.com/1235936
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Anders Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592784}
2 files changed