Use an enum class instead of a bool for readability.

Bug: angleproject:4888
Change-Id: I5fddec4ccc1cb7e28940c0566979f5fd5e051d60
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2702652
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Charlie Lao <cclao@google.com>