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

This reverts commit a73323d3.

Bug: 
Change-Id: I2f17b1b121380ec3d70f42d7cd0015720d6dc7d1
Reviewed-on: https://chromium-review.googlesource.com/518132Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45611}