Add variable set/get utilities on CSSVariableResolver.

R=futhark@chromium.org

Bug: 641877
Change-Id: I0b4dc43b29f7e30dbc3eab89f22ffa5505e6a333
Reviewed-on: https://chromium-review.googlesource.com/1235999
Commit-Queue: Anders Ruud <andruud@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592788}
2 files changed