Transient local storage namespaces need to ref their storage areas
https://bugs.webkit.org/show_bug.cgi?id=140006
<rdar://problem/19225389>

Reviewed by Sam Weinig.

We need to keep transient storage areas around even if they have no active connections,
so use a RefPtr instead of a raw pointer. This matches what we do for session storage namespaces.

* UIProcess/Storage/StorageManager.cpp:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@177823 268f45cc-cd09-0410-ab3c-d52691b4dbfc