[webcrypto] Remove a lingering usage of RSA-ES.

TBR=jww
BUG=372920,245025

Review URL: https://codereview.chromium.org/344503003

git-svn-id: svn://svn.chromium.org/blink/trunk@176389 bbb929c8-8fbe-4397-9dbb-9b2b20218538