It was incorrectly calling the pbuffer function.

BUG=angleproject:891

Change-Id: I6959540262a8ef80886e41da795f9e4aff218535
Reviewed-on: https://chromium-review.googlesource.com/299441Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>