Don't roundtrip to JS to update the length. This is 1) faster (although we don't care) and 2) avoids stackchecks that otherwise make the .Assert fail on stack overflow.

BUG=chromium:502147
LOG=n

Review URL: https://codereview.chromium.org/1189423002

Cr-Commit-Position: refs/heads/master@{#29142}