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While, at the moment, I don't believe we will end up asking for the
authenticator's ECDH key twice, when we support the PRF extension we
could. Thus add a cache and skip asking an authenticator for it twice.

Change-Id: I73ecaa38447cbbe04f34c12d83f9c79bca553c90
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2255208
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@{#781110}