Move platform specific information to PlatformConfiguration class.

Author: gspencergoog

lib/ui/window/window.h

@@ -22,6 +22,8 @@ class Window final {
 
   ~Window();
 
+  const uint64_t get_window_id() const { return viewport_metrics_.view_id; }
+
   const ViewportMetrics& viewport_metrics() const { return viewport_metrics_; }
 
   void UpdateWindowMetrics(const tonic::DartPersistentValue& library,