Make createWindow (mostly) work with OOPIF

Source/platform/PlatformScreen.cpp

Index: Source/platform/PlatformScreen.cpp
diff --git a/Source/platform/PlatformScreen.cpp b/Source/platform/PlatformScreen.cpp
index 60b2ee3b8ba9518e73b1c6e9692fb4594a4b9940..dcfd0e4539e09afac93b1be341a2c19d5f6306f0 100644
--- a/Source/platform/PlatformScreen.cpp
+++ b/Source/platform/PlatformScreen.cpp
@@ -80,12 +80,9 @@ IntRect screenRect(Widget* widget)
     return IntRect(hostWindow->screenInfo().rect);
 }
 
-IntRect screenAvailableRect(Widget* widget)
+IntRect screenAvailableRect(const HostWindow& hostWindow)
 {
-    HostWindow* hostWindow = toHostWindow(widget);
-    if (!hostWindow)
-        return IntRect();
-    return IntRect(hostWindow->screenInfo().availableRect);
+    return IntRect(hostWindow.screenInfo().availableRect);
 }
 
 uint16_t screenOrientationAngle(Widget* widget)