Refactor packed enum generation to support EGL enums. 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>