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The MakeCredential and GetAssertion calls take a large number of
arguments. This change makes them structs instead. This will be useful
in the next change when these arguments need to be held pending a
notification on Android.

BUG=1002262

Change-Id: Ia03c2f786e36243902749183e6d3307a45c0717a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2500275
Commit-Queue: Adam Langley <agl@chromium.org>
Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#822731}