This is in preparation for turning the flag off by default.

BUG=chromium:552100
LOG=n

Review URL: https://codereview.chromium.org/1511293002

Cr-Commit-Position: refs/heads/master@{#32729}

This avoids touching the Array prototype, which may have been tampered with.

BUG=chromium:443982
LOG=n

Review URL: https://codereview.chromium.org/820503005

Cr-Commit-Position: refs/heads/master@{#25908}