LibWeb: Make Web::PageClient GC-allocated

This is a first step towards simplifying the ownership model of
Web::Page. Soon Web::Page will store its WebClient as a
NonnullGCPtr to help solve lifetime issues of the client being
destroyed before the page.

Userland/Services/WebWorker/ConnectionFromClient.cpp

@@ -36,7 +36,7 @@ void ConnectionFromClient::request_file(Web::FileRequest request)
 
 ConnectionFromClient::ConnectionFromClient(NonnullOwnPtr<Core::LocalSocket> socket)
     : IPC::ConnectionFromClient<WebWorkerClientEndpoint, WebWorkerServerEndpoint>(*this, move(socket), 1)
-    , m_page_host(PageHost::create(*this))
+    , m_page_host(PageHost::create(Web::Bindings::main_thread_vm(), *this))
 {
 }