[HTTP] Make sure active HTTPRequest objects are not deleted.

(HttpCache, HttpConnectionManager, HttpRequest) : Mange HTTPRequest objects using std::shared_ptr
src (WindowImp::~WindowImp, WindowImp::preload) : Refine
src (WindowProxy::WindowProxy, WindowProxy::poll, WindowProxy::navigate) : Refine
src/css (Box::backgroundRequest) : Refine
src/css (Block::resolveBackground) : Refine
src/css (CSSListStyleImageValueImp) : Refine
src/css (TableWrapperBox::layOutTableBox) : Refine
src/css (ViewCSSImp::preload) : Refine
src/html (HTMLImageElementImp::handleMutation, HTMLImageElementImp::notify) : Refine
src/html (HTMLLinkElementImp::refresh, HTMLLinkElementImp::linkStyleSheet, HTMLLinkElementImp::linkIcon) : Refine
src/html (HTMLObjectElementImp::refresh, HTMLObjectElementImp::handleRefresh) : Refine
src/html (HTMLReplacedElementImp) : Refine
src/html (HTMLScriptElementImp::prepare) : Refine
src/HTTPRequest.test.cpp : Refine