[webcrypto] Make WebCryptoKey nullable.

This is a convenience for the embedder so it is easier to use as an out-parameter like:

  WebCryptoKey key = WebCryptoKey::createNull();
  if (doSomethingThatMightFailOrCreateKey(&key)) {
    ..
  }

On the other side of the Blink API, WebCryptoKey is never expected to be null.

BUG=245025

Review URL: https://codereview.chromium.org/48753003

git-svn-id: svn://svn.chromium.org/blink/trunk@160989 bbb929c8-8fbe-4397-9dbb-9b2b20218538