<rdar://problem/3885708> save memory in icon DB by not using NSSets when holding...

	<rdar://problem/3885708> save memory in icon DB by not using NSSets when holding 1 object

        Reviewed by sullivan.

        * Misc.subproj/WebIconDatabase.m:
        (+[WebIconDatabase sharedIconDatabase]): added timing code
        (-[WebIconDatabase _clearDictionaries]): new
        (-[WebIconDatabase _loadIconDictionaries]): call _clearDictionaries in 2 places before we bail, use _web_setObjectUsingSetIfNecessary:forKey: when adding site URLs to the iconURLToURLs dictionary
        (-[WebIconDatabase _updateFileDatabase]): fixed comment
        (-[WebIconDatabase _setIconURL:forURL:]): use _web_setObjectUsingSetIfNecessary:forKey: when adding site URLs to the iconURLToURLs dictionary
        (-[WebIconDatabase _releaseIconForIconURLString:]): handle NSString objects retured from iconURLToURLs
        (-[NSMutableDictionary _web_setObjectUsingSetIfNecessary:forKey:]): new, puts a set on the dictionary when there are 2 or more object for s key

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@8034 268f45cc-cd09-0410-ab3c-d52691b4dbfc