Summary:
This avoids an intermittent reentrancy bug in JSC on iOS 11
(https://bugs.webkit.org/show_bug.cgi?id=186827). It also makes the
code more consistent with android.
Reviewed By: amnn
Differential Revision: D13313265
fbshipit-source-id: f42476b2f660e127ecfc9c72584554817eea1010