Port 5d38d681

Original commit message:
    It didn't support subclassing case at all and in non-subclassing case the runtime
    allocation didn't do the slack tracking step.

R=ishell@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=chromium:563339
LOG=N

Review URL: https://codereview.chromium.org/1493553007

Cr-Commit-Position: refs/heads/master@{#32591}