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Authenticator{MakeCredential,GetAssertion}Blocking meant to free the
unmanaged pointers returned by Windows by wrapping them in unique_ptrs.
But they assigned nullptr, and the actual pointers were never freed.

Also make FakeWinWebAuthnApi ensure callers free all returned pointers
before it is destructed. Also fix some doc strings.

This is a follow-up to comments in
https://chromium-review.googlesource.com/c/chromium/src/+/2197475.

Fixed: 1081450, 1084698
Change-Id: Ifc4d228a63f679f1f3cd2f2f75b11c49b4af8d3c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2207736
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Reviewed-by: Nina Satragno <nsatragno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770360}