Remove the WebCrypto::digestSynchronous() method.

It has been replaced by the createDigestor() method and is no longer called.

BUG=349514

Review URL: https://codereview.chromium.org/243383003

git-svn-id: svn://svn.chromium.org/blink/trunk@171968 bbb929c8-8fbe-4397-9dbb-9b2b20218538