This probably was left out accidentally. Also this fixes the return
type of the prototype in the extension header (should be void, not
boolean).

BUG=angleproject:1523

Change-Id: I7bf0b36b05a4cba4cb6fb2411fc53103dda54bfe
Reviewed-on: https://chromium-review.googlesource.com/657898Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>