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This is a reland of 7acb4716 with a fix.

In the original CL, CredentialMetadata::HmacForStorage wrapped
HMAC::Init(base::StringPiece) in a DCHECK. In non-debug this resulted in
HMAC::Sign getting called on an unintialized HMAC instance.

Change-Id: Ibf6e34dc381812254aaed5c20a83eb6a28d43246
Reviewed-on: https://chromium-review.googlesource.com/1087315Reviewed-by: Balazs Engedy <engedy@chromium.org>
Reviewed-by: Adam Langley <agl@chromium.org>
Commit-Queue: Balazs Engedy <engedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#564864}