LibWeb/HTML: Move WindowOrWorkerGlobalScope code into a mixin class

This will allow us to easily share the implementations of these methods
between Window and WorkerGlobalScope. The mixin class mirrors what I
already did for the Fetch API's Body mixin in 5ad62833.

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -303,6 +303,7 @@ set(SOURCES
     HTML/Timer.cpp
     HTML/Window.cpp
     HTML/WindowEventHandlers.cpp
+    HTML/WindowOrWorkerGlobalScope.cpp
     HTML/WindowProxy.cpp
     HTML/Worker.cpp
     HTML/WorkerDebugConsoleClient.cpp