[webcrypto] Add AES-KW to the list of registered algorithms.

This algorithm is not yet defined in the WebCrypto spec, however is planned for inclusion and should make a forthcoming spec update.

BUG=245025

Review URL: https://codereview.chromium.org/111603004

git-svn-id: svn://svn.chromium.org/blink/trunk@163662 bbb929c8-8fbe-4397-9dbb-9b2b20218538