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

Review URL: https://codereview.chromium.org/757873003

Cr-Commit-Position: refs/heads/master@{#306667}
13 files changed