Map::AsElementsKind returns the given map if it already has the desired
elements kind.

Change-Id: Ia9c92eabdb28c82da376eb87f0117dc76414240d
Reviewed-on: https://chromium-review.googlesource.com/1164368Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54941}