[EME][Mac] SecureStop left on disk in Private Browsing mode.

[EME][Mac] SecureStop left on disk in Private Browsing mode.
https://bugs.webkit.org/show_bug.cgi?id=175162

Reviewed by Eric Carlson.

Return an empty string from mediaKeysStorageDirectory() when the page indicates that storage should
be ephemeral(). Previously, an empty string in this case would be treated as an error. Instead, treat
an empty string as valid, and do not try to store or retrieve session information to disk in that case.

* Modules/encryptedmedia/legacy/WebKitMediaKeySession.cpp:
(WebCore::WebKitMediaKeySession::mediaKeysStorageDirectory const):
* platform/graphics/avfoundation/objc/CDMSessionAVContentKeySession.mm:
(WebCore::CDMSessionAVContentKeySession::releaseKeys):
(WebCore::CDMSessionAVContentKeySession::update):
(WebCore::CDMSessionAVContentKeySession::generateKeyReleaseMessage):
(WebCore::CDMSessionAVContentKeySession::contentKeySession):
* platform/graphics/avfoundation/objc/CDMSessionMediaSourceAVFObjC.mm:
(WebCore::CDMSessionMediaSourceAVFObjC::storagePath const):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@220247 268f45cc-cd09-0410-ab3c-d52691b4dbfc