This test documents the basic behavior of the EnumCache which is shared
on the DescriptorArray.

Change-Id: Idd40670d99d81bb5e4b6161ffc47f2898ca9d2a9
Reviewed-on: https://chromium-review.googlesource.com/643297
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47735}