Reviewed by Maciej.

	- fixed most likely cause of 3125565 -- 2% regression running the PLT for uncached loads

        * Misc.subproj/WebNSUserDefaultsExtras.m:
        (+[NSUserDefaults _web_defaultsDidChange]): Added. Clears out cache when defaults change.
        (+[NSUserDefaults _web_addDefaultsChangeObserver]): Added. Sets up the above to run at the right time.
        (+[NSUserDefaults _web_acceptLanguageHeaderForPreferredLanguages]): Changed to cache the result in a
	global and regenerate it when the global is cleared.
        (+[NSUserDefaults _web_preferredLanguageCode]): Ditto.

WebKit:

        Reviewed by Maciej.

        - fixed most likely cause of 3125565 -- 2% regression running the PLT for uncached loads

        Don't recompute the user agent when it doesn't change. It almost never changes.

        * WebView.subproj/WebController.m:
        (-[WebController initWithView:controllerSetName:]): Add observer so we know when defaults change.
        (-[WebController dealloc]): Remove observer.
        (-[WebController setApplicationNameForUserAgent:]): Clear out computed user agent to force it
	to be recomputed later.
        (-[WebController applicationNameForUserAgent]): Just retain since we copied when we stored it
	so we know it's not mutable.
        (-[WebController customUserAgent]): Ditto.
        (-[WebController userAgentForURL:]): Use the cached user agent if it's good. Otherwise compute
	and cache the user agent string. This means that we will almost never recompute it.

        * WebView.subproj/WebControllerPrivate.h: Add userAgent field to cache in.
        * WebView.subproj/WebControllerPrivate.m:
        (-[WebControllerPrivate dealloc]): Release userAgent.
        (-[WebController _defaultsDidChange]): Release and nil userAgent.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@3019 268f45cc-cd09-0410-ab3c-d52691b4dbfc