Fixed http://bugs.webkit.org/show_bug.cgi?id=12900 Page tear-down 
        forces garbage collection once per frame

        Also fixed <rdar://problem/5286989> GC on window close does not always 
        bring the JS object count down to 0
        
        Implemented a 0-delay GC timer in WebCore. Instead of forcing GC
        immediately, code that thinks it has created a lot of garbage starts
        the timer. This has two advantages: 

        1) Multiple GCs can coalesce. In my pathological test case, this 
        improves performance by an order of magnitude. 
        
        2) Conservative marking is less likely to keep alive important dead
        objects, like the window object, because the stack is small and free of
        JS processing when the timer fires.

        Added GCController.h/.cpp, sometimes blindly:
        * WebCore.pro:
        * WebCore.vcproj/WebCore.vcproj:
        * WebCore.xcodeproj/project.pbxproj:
        * WebCoreSources.bkl:

        Added singleton that implements GC on a 0-delay timer:
        * bindings/js/GCController.h: Added.
        * bindings/js/GCController.cpp: Added.
        
        Changed Collector::collect() calls below to call to the singleton:
        * bindings/js/kjs_proxy.cpp:
        (WebCore::KJSProxy::~KJSProxy):
        * bindings/js/kjs_window.cpp:
        (KJS::Window::clear):
        * history/CachedPage.cpp:
        (WebCore::CachedPage::clear):
        
        * page/Frame.cpp:
        (WebCore::Frame::~Frame): Removed previous slightly hackish attempt to
        avoid conservative marking of the window object.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@24493 268f45cc-cd09-0410-ab3c-d52691b4dbfc