Retrieve length from the buffer instead of storing in as value in the
map.

Change-Id: Icdc87bb46c29fff779bf6e98139324662ced5bf6
Reviewed-on: https://chromium-review.googlesource.com/518171
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45600}