All major browsers except Safari have shipped it without the prefix (see http://caniuse.com/#feat=cryptography). Is there a reason to keep the webkit prefix still?
WebKit implements an early version of the WebCrypto spec, different from other browsers that added this feature later. Because of this, exposing it was crypto.subtle would be a bad idea.
What we need to do is to implement the up to date spec, and to expose that as crypto.subtle.
*** This bug has been marked as a duplicate of bug 160880 ***