remove byte-accounting assert for crbug.com/536163

remove byte-accounting assert for crbug.com/536163

The change https://codereview.chromium.org/1274453002 is causing a breakage
on the Chrome OS builds. It is not clear how to easily revert because there is
a following on dependent change (https://codereview.chromium.org/1351983002) am
I am not too familiar with the revert process.

The simplest revert is to remove the DCHECK.

BUG=chromium:536163
LOG=n
NOTRY=true
R=machenbach@chromium.org, cmumford@chromium.org, hpayer@chromium.org

Review URL: https://codereview.chromium.org/1372693002 .

Cr-Commit-Position: refs/heads/master@{#30959}