Check that usage isn't empty when generateKey() is called

Check that usage isn't empty when generateKey() is called

This is my second attempt at https://codereview.chromium.org/745443002/,
after it was reverted. The broken test that caused it to get reverted was
fixed in https://codereview.chromium.org/757813002/.

BUG=425645
Committed: https://crrev.com/70f820af8eb1f642a0a5e7df8dd4c3a3c269de29
Cr-Commit-Position: refs/heads/master@{#306667}

[nharper, 2014-11-26 18:46:04]

nharper@chromium.org changed reviewers:
+ eroman@chromium.org, pdr@chromium.org

[nharper, 2014-11-26 18:46:04]

I attempted to run the mac_blink_rel try job, but its result was an exception.
If I run the crypto layout tests on my mac with this cl patched in, the tests
pass. I could use some help figuring out why it's not passing when I git cl try
it.

[eroman, 2014-11-26 20:10:15]

LGTM.

That bot might just be broken. I sent it to some other mac blink trybots let's
see if they pass.

[eroman, 2014-11-26 20:55:58]

The test passed on mac_blink_dbg, I consider that sufficient proof.

[eroman, 2014-11-26 20:56:05]

The CQ bit was checked by eroman@chromium.org

[commit-bot: I haz the power, 2014-11-26 20:57:23]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/757873003/1

[commit-bot: I haz the power, 2014-11-26 21:48:07]

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

[commit-bot: I haz the power, 2014-11-26 21:48:08]

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

[nharper, 2014-12-03 19:13:37]

The CQ bit was checked by nharper@chromium.org

[commit-bot: I haz the power, 2014-12-03 19:14:37]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/757873003/20001

[commit-bot: I haz the power, 2014-12-03 20:36:18]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2014-12-03 20:36:52]

Patchset 2 (id:??) landed as
https://crrev.com/70f820af8eb1f642a0a5e7df8dd4c3a3c269de29
Cr-Commit-Position: refs/heads/master@{#306667}