This avoids creating unnecessary intermediate maps.

BUG=chromium:588893

Change-Id: Ibc20908270a6fab58fc391f4b7e7941d9c043e9b
Reviewed-on: https://chromium-review.googlesource.com/445785Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43486}