(ECMAScriptContext::getCurrentContext): New
Switch over to the V8 context model from the activation model

(WindowImp::enter, WindowImp::exit): New
(WindowImp::activate): Remove
(WindowImp::poll): Switch over to the enter/exit model
(DocumentWindow::enter, DocumentWindow::exit): New
(DocumentWindow::activate): Remove
(DocumentWindow::setDocument): Refine
(DocumentImp::enter, DocumentImp::exit, DocumentImp::getContext): New
(DocumentImp::activate): Remove
(DocumentImp::setEventHandler): Refine
(ECMAScriptContext::enter, ECMAScriptContext::exit): New
(ECMAScriptContext::activate): Remove
(EventTargetImp::dispatchEvent): Switch over to the enter/exit model
(HTMLElementImp::xblEnteredDocument, HTMLElementImp::invokeShadowTarget): Switch over to the enter/exit model
(HTMLElementImp::handleMutation, HTMLElementImp::setEventHandler): Refine
(HTMLBodyElementImp::handleMutation, HTMLMarqueeElementImp::handleMutation, HTMLScriptElementImp::execute): Refine