[PopupControl] PopupControl.CSSBridge.setSkinClassNameMethod deleted

While reviewing an unrelated change I discovered an incompatible API change that appears to be a side effect from RT-35175.

class javafx/scene/control/PopupControl$CSSBridge contains the following method in FX 8:
    protected void setSkinClassName(java.lang.String);

but that method was removed as part of RT-35175. This breaks both source and binary compatibility.