Allow to get key from SPAKE2 after the first round trip.
The second round trip is optional and is not the part of SPAKE2 algorithm.
Application may want to get key and do verification with usefull payload.

BUG=438470
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Review URL: https://codereview.chromium.org/761663004

Cr-Commit-Position: refs/heads/master@{#306844}
3 files changed