Convert the very simple EGL texture type enum.

BUG=angleproject:1618

Change-Id: Ieea382a282a8f2544f2982627e8445e6e5cea826
Reviewed-on: https://chromium-review.googlesource.com/1019386
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>