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Some Proto messages are sent to Apiary endpoints, which translate Proto
enums to strings instead of leaving them as enums. Though this doesn't
interact normally with Protos, this behavior has been present in Apiary
from the beginning and is not considered a bug. See this change for
details: https://critique.corp.google.com/#review/187680679.

Thus, when communicating with those endpoints, the proto values should
be converted from enums to strings when send messages to Apiary and
should be converted from strings to enumsm when receiving messages from
Apiary.

This CL adds these conversion functions and integrates them throughout
CryptAuth.

Bug: 824568
Change-Id: Icc2367a6a325fe9da42905f9456d2b2103c5eb25
Reviewed-on: https://chromium-review.googlesource.com/1145588
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Jeremy Klein <jlklein@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577027}