This was leading to a resouce leak with fast unpack.

BUG=angleproject:1513

Change-Id: I4ff6c6e3c889392e314654d63f2db8f671b6913c
Reviewed-on: https://chromium-review.googlesource.com/383711
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>