GitLab

- As a follow-up of https://crrev.com/c/1841451, return the ECPrivateKey
  directly to avoid unnecessary hashing

Bug: 1010968
Change-Id: Iff13d775f511ba031eba33d63e4d47403d205280
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1863297
Commit-Queue: Alex Chau <alexchau@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706396}