The code which pushes and pops to the function name inference stack
generally checks if the stack is active with the IsOpen method. One
piece of code pertaining to async functions was missing that check.
This patch adds it.

BUG=chromium:658267
R=gsathya,caitp

Review-Url: https://codereview.chromium.org/2514893002
Cr-Commit-Position: refs/heads/master@{#41120}