Move closing logic to api::BrowserWindow

Closing a BrowserWindow is essentially closing a WebContents, the logic
should not be in NativeWindow.

atom/browser/api/atom_api_web_contents.h

@@ -54,6 +54,7 @@ namespace api {
 // dispatch those events.
 class ExtendedWebContentsObserver {
  public:
+  virtual void OnCloseContents() {}
   virtual void OnRendererResponsive() {}
 };