[webcrypto] Fix the key usages used by some layout tests.

A Chromium-side change is going to make these illegal, so update the tests first.

Once the Chromium side has landed, additional tests for error case will be added.

BUG=372944,245025

Review URL: https://codereview.chromium.org/296723006

git-svn-id: svn://svn.chromium.org/blink/trunk@174522 bbb929c8-8fbe-4397-9dbb-9b2b20218538