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/1171504Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>