Expose +[WebStorageManager _storageDirectoryPath] as SPI

Expose +[WebStorageManager _storageDirectoryPath] as SPI
https://bugs.webkit.org/show_bug.cgi?id=68951

Reviewed by Brady Eidson.

This patch exposes +[WebStorageManager _storageDirectoryPath], which
pulls the path from NSUserDefaults if the preference is set. Otherwise,
it falls back on the default path.

* Storage/WebStorageManager.mm:
(+[WebStorageManager _storageDirectoryPath]): Adding static variable
so that we don't reread user defaults repeatedly.
(WebKitInitializeStorageIfNecessary):
* Storage/WebStorageManagerPrivate.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@96255 268f45cc-cd09-0410-ab3c-d52691b4dbfc