Add unit test covering ax_enum_util and fix issues found.

TBR=tsepez@chromium.org

Change-Id: I9ae1d2d051ab5a045c3987b781907f684050a3bf
Reviewed-on: https://chromium-review.googlesource.com/c/1460490
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Akihiro Ota <akihiroota@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631484}
5 files changed