Expose a constructor for WebCryptoAlgorithm.

This allows unittests on the chromium-side to construct it.

BUG=245025

Review URL: https://chromiumcodereview.appspot.com/21185005

git-svn-id: svn://svn.chromium.org/blink/trunk@155223 bbb929c8-8fbe-4397-9dbb-9b2b20218538