This enforces that its enumeration values fit in a byte, as required
by Map's {instance_type} field (and probably other parts of the
system).

Clang helpfully emits this error message if an enum value goes out
of range:

   enumerator value 256 is not representable in the underlying type 'uint8_t' (aka 'unsigned char')

Change-Id: I533cd5afc755e7163c2fd40f7b00d9adfd960895
Reviewed-on: https://chromium-review.googlesource.com/565892Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46544}