Removed un-needed forward declare of nsIBrowserWindow.

widget/src/support/nsWidgetSupport.h

@@ -42,7 +42,6 @@ class nsICheckButton;
 class nsIScrollbar;
 class nsITooltipWidget;
 class nsITextWidget;
-class nsIBrowserWindow;
 
 // These are a series of support methods which help in the creation
 // of widgets. They are not needed, but are provided as a convenience