Implement WebPlatformStrategies::transientLocalStorageNamespace
https://bugs.webkit.org/show_bug.cgi?id=115876
<rdar://problem/13852871>

Reviewed by Beth Dakin.

* WebProcess/WebCoreSupport/WebPlatformStrategies.cpp:
(WebKit::WebPlatformStrategies::transientLocalStorageNamespace):
Return a session storage namespace for a random page when UI process storage is enabled,
otherwise call the base class implementation.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149847 268f45cc-cd09-0410-ab3c-d52691b4dbfc