The context pointer was throwing off the count by one.
Also change the type to void * for ease of use.

BUG=angleproject:1892
BUG=chromium:678870

Change-Id: Iffac667f0aa896d9247e9ffddfeb3bc447a9692f
Reviewed-on: https://chromium-review.googlesource.com/441931Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>