LibWeb: Defer the handling of WebContent process crashes

Handling crashes synchronously is finicky since we're modifying the
m_client_state struct while in a callback lambda owned by it.
Let's avoid all the footguns here by simply using deferred_invoke()
and handling the crash on next event loop iteration instead.

Userland/Libraries/LibWeb/OutOfProcessWebView.h

@@ -90,6 +90,8 @@ private:
     void create_client();
     WebContentClient& client();
 
+    void handle_web_content_process_crash();
+
     URL m_url;
 
     struct ClientState {