Reviewed by Mitz Pettel RTL

        Preparation for upcoming work making LocalStorage persistent.

        StorageMaps normally have copy-on-write semantics to help support SessionStorage.
        For LocalStorage, we never want this behavior.  When we forcefully import items into 
        a StorageMap from the LocalStorage background thread, this new import method will be used.

        * storage/StorageMap.cpp:
        (WebCore::StorageMap::importItem): Add a deep-copy of the item to the map without worrying
          about copy-on-write.
        * storage/StorageMap.h:



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@32897 268f45cc-cd09-0410-ab3c-d52691b4dbfc