[EME][Mac] Support secure proof of key release in the com.apple.fps.2_0 keysystem.
https://bugs.webkit.org/show_bug.cgi?id=136898

Reviewed by Eric Carlson.

Add support for proof of key release to the CDM by supporting a new "mime" type called
"keyrelease". When a session of type "keyrelease" is created, the initData is the server
certificate, the keyMessage contains the first stored proof of key release if present,
and closing the session will remove that proof from storage.

* platform/graphics/avfoundation/CDMPrivateMediaSourceAVFObjC.mm:
(WebCore::CDMPrivateMediaSourceAVFObjC::supportsKeySystemAndMimeType):
* platform/graphics/avfoundation/objc/CDMSessionMediaSourceAVFObjC.h:
* platform/graphics/avfoundation/objc/CDMSessionMediaSourceAVFObjC.mm:
(WebCore::CDMSessionMediaSourceAVFObjC::CDMSessionMediaSourceAVFObjC):
(WebCore::CDMSessionMediaSourceAVFObjC::generateKeyRequest):
(WebCore::CDMSessionMediaSourceAVFObjC::releaseKeys):
(WebCore::sessionStorageDirectory):
(WebCore::CDMSessionMediaSourceAVFObjC::update):
(WebCore::CDMSessionMediaSourceAVFObjC::generateKeyReleaseMessage):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@173791 268f45cc-cd09-0410-ab3c-d52691b4dbfc