[infra-sk] Add sass_library target for themes
[infra-sk] Add sass_library target for themes

Will be useful for apps converting to bazel

BUG: skia:12536
Change-Id: Iea4f607f61d9f666425e3a03cfdc8cbde9087894
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/458922
Reviewed-by: Leandro Lovisolo <f558143c2568be8d55575d13f30a1fc5cb8009d5@google.com>
Commit-Queue: Ravi Mistry <9fa2e7438b8cb730f96b74865492597170561628@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot