LibWeb+WebContent: Spawn Worker processes from the chrome

Instead of spawning these processes from the WebContent process, we now
create them in the Browser chrome.

Part 1/N of "all processes are owned by the chrome".

Userland/Libraries/LibWeb/Forward.h

@@ -457,6 +457,7 @@ class Window;
 class WindowEnvironmentSettingsObject;
 class WindowProxy;
 class Worker;
+class WorkerAgent;
 class WorkerDebugConsoleClient;
 class WorkerEnvironmentSettingsObject;
 class WorkerGlobalScope;