Make the state setting functions of StateManagerGL public.

BUG=angleproject:1113

Change-Id: Ic435c06ece24064a2f2403119fa30309aba37608
Reviewed-on: https://chromium-review.googlesource.com/295240Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>