[WebCrypto] Replace KeyPair with an Object.

This corresponds with the spec update:
https://dvcs.w3.org/hg/webcrypto-api/rev/309495c2794e

Which changes KeyPair --> CryptoKeyPair (of type Dictionary).

BUG=386757

Review URL: https://codereview.chromium.org/333293004

git-svn-id: svn://svn.chromium.org/blink/trunk@176558 bbb929c8-8fbe-4397-9dbb-9b2b20218538