[webcrypto] Add the KeyAlgorithm interface.

This matches the changes in https://dvcs.w3.org/hg/webcrypto-api/rev/2fa3494f0179

 * Key.algorithm is now reflected as a KeyAlgorithm interface rather than Algorithm
 * HMAC no longer takes the hash as an operational parameter (key move to import/generation)
 * RSA-SSA no longer takes the hash as an operation (move to key import/generation)parameter

BUG=245025

Review URL: https://codereview.chromium.org/179353002

git-svn-id: svn://svn.chromium.org/blink/trunk@167979 bbb929c8-8fbe-4397-9dbb-9b2b20218538