Allow an extra slot in PackedEnumMap. This can be used for fast lookups with InvalidEnum. Will be useful for optimizing valid primitive mode lookups. Bug: angleproject:2747 Change-Id: I5d668863eb23d3c1eb85a775b3e9e46f25a0a335 Reviewed-on: https://chromium-review.googlesource.com/1171504 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Frank Henigman <fjhenigman@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>