WebCrypto: properly normalize optional numeric parameters.

This addresses a FIXME, and also improves an exception message.

BUG=245025

Review URL: https://chromiumcodereview.appspot.com/23361019

git-svn-id: svn://svn.chromium.org/blink/trunk@156675 bbb929c8-8fbe-4397-9dbb-9b2b20218538