Allow WK2 to read from NSUserDefaults when it has no prefs identifier

Allow WK2 to read from NSUserDefaults when it has no prefs identifier
https://bugs.webkit.org/show_bug.cgi?id=128124

Reviewed by Sam Weinig.

WebPreferences would never read from NSUserDefaults if no identifier
had been set, but this prevents us from reading useful debugging prefs.

Fix to read pref names using a "WebKit2" prefix when no identifier
is set.

* UIProcess/mac/WebPreferencesMac.mm:
(WebKit::makeKey):
(WebKit::WebPreferences::platformInitializeStore):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@163336 268f45cc-cd09-0410-ab3c-d52691b4dbfc