Allow to get key from SPAKE2 after the first round trip.

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
R=agl@chromium.org

Committed: https://chromium.googlesource.com/chromium/src/+/fb2ccf63152e415e5bb1243b1284ef496cf45c03

[Vitaly Buka (NO REVIEWS), 2014-12-04 02:06:36]

vitalybuka@chromium.org changed reviewers:
+ agl@chromium.org, gene@chromium.org, sergeyu@chromium.org

[Vitaly Buka (NO REVIEWS), 2014-12-04 02:12:36]

vitalybuka@chromium.org changed reviewers:
+ rvargas@chromium.org
- sergeyu@chromium.org

[Vitaly Buka (NO REVIEWS), 2014-12-04 02:12:45]

Please review.

[agl, 2014-12-04 02:29:18]

lgtm

[Vitaly Buka (NO REVIEWS), 2014-12-04 08:07:41]

The CQ bit was checked by vitalybuka@chromium.org

[commit-bot: I haz the power, 2014-12-04 08:08:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/761663004/20001

[commit-bot: I haz the power, 2014-12-04 08:11:58]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-12-04 08:11:59]

Try jobs failed on following builders:
  chromium_presubmit on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/chromium_presub...)

[Vitaly Buka (NO REVIEWS), 2014-12-04 16:16:56]

The CQ bit was checked by vitalybuka@chromium.org

[Vitaly Buka (NO REVIEWS), 2014-12-04 16:17:39]

The CQ bit was unchecked by vitalybuka@chromium.org

[commit-bot: I haz the power, 2014-12-04 17:18:08]

Patchset 2 (id:??) landed as
https://crrev.com/fb2ccf63152e415e5bb1243b1284ef496cf45c03
Cr-Commit-Position: refs/heads/master@{#306844}

[Vitaly Buka (NO REVIEWS), 2014-12-04 17:18:12]

Committed patchset #2 (id:20001) manually as fb2ccf6.