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This change causes |AuthenticatorImpl| to only serialize
|CollectedClientData| once for each request. Since the code then carries
it around as a string, the |CollectedClientData| object itself is never
used except to immediately call |SerializeToJson| on it. Thus
|CollectedClientData| is folded into a function on |AuthenticatorImpl| to
save two indirections.

Along the way, implement the changes to the Token Binding member[1] and
re-add the random hint to not template match[2] now that it doesn't
break things.

[1] https://github.com/w3c/webauthn/commit/a47fe1c4d53b123caa7abc76e9659b95dc1c1a16
[2] https://chromium.googlesource.com/chromium/src/+/eb234c8e7356f2239700c604ad7b3eda42aaac33

Bug: 814103
Change-Id: Ifa96f3eee11bcbf740c7adff43e092ce5361210a
Reviewed-on: https://chromium-review.googlesource.com/929807Reviewed-by: Balazs Engedy <engedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#538860}