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Enums can be unsigned or signed. This change corrects an unsigned/signed
comparison error by making sure this enum is always unsigned.

Bug: 1141421
Change-Id: Ic3576497c77fd6e3391e6d42a4653c699f337a6d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2493201Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Richard Townsend <richard.townsend@arm.com>
Cr-Commit-Position: refs/heads/master@{#820057}