Use EVP_DigestSign* rather than EVP_Sign* in SignatureCreator.

Use EVP_DigestSign* rather than EVP_Sign* in SignatureCreator.

Avoids having to hold on to the key separately. EVP_DigestSign
is tidier about bounds checks and maximum buffer size. Also
account for the return values having been normalized.

BUG=none

Committed: https://crrev.com/183ce6363d441d4d5c46682d76dc774166727105
Cr-Commit-Position: refs/heads/master@{#312357}

[davidben, 2015-01-20 19:15:38]

davidben@chromium.org changed reviewers:
+ rsleevi@chromium.org

[davidben, 2015-01-20 19:15:38]

Had this lying around. Just an inconsequential bit of cleanup.

[Ryan Sleevi, 2015-01-21 01:14:59]

The CQ bit was checked by rsleevi@chromium.org

[Ryan Sleevi, 2015-01-21 01:15:00]

lgtm

[commit-bot: I haz the power, 2015-01-21 01:16:32]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/382633009/40001

[commit-bot: I haz the power, 2015-01-21 14:22:05]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2015-01-21 14:22:59]

Patchset 3 (id:??) landed as
https://crrev.com/183ce6363d441d4d5c46682d76dc774166727105
Cr-Commit-Position: refs/heads/master@{#312357}