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The initial steps of both makeCredential and getAssertion are the same.
This change pulls that behaviour out of makeCredential and call it for
both operations.

Change-Id: If10dac3dce6c5dd4d1080c80730066afd5365b0c
Reviewed-on: https://chromium-review.googlesource.com/c/1461216
Commit-Queue: Adam Langley <agl@chromium.org>
Auto-Submit: Adam Langley <agl@chromium.org>
Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#630535}