The special enum value 'EnumCount' is used by EnumSize() to determine
how large to make the PackedEnumMap::mPrivateData array. However, that
enum entry was being assigned an incorrect value, leading to the array
being constructed too small and causing a crash in PackedEnumMap's
std::initializer_list constructor when it went beyond the size of the
array to assign each of the necessary values.

The fix is to remove the assignment of CacheOpResult::EnumCount and let
it indicate the real number of entries in the enum.

Bug: angleproject:4393
Test: end2end
Change-Id: If9635c61e72aadc6527d416f83be18add8b24572
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2050026Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>