b=581771 report GTK themed tooltips as opaque r=roc a=joe

widget/src/gtk2/nsNativeThemeGTK.cpp

@@ -1407,6 +1407,8 @@ nsNativeThemeGTK::GetWidgetTransparency(nsIFrame* aFrame, PRUint8 aWidgetType)
   case NS_THEME_MENUPOPUP:
   case NS_THEME_WINDOW:
   case NS_THEME_DIALOG:
+  // Tooltips use gtk_paint_flat_box().
+  case NS_THEME_TOOLTIP:
     return eOpaque;
   }