Bug 1447788 - implement SetNonClientMargins as a noop returning NS_OK in HeadlessWidget.h, r=bdahl.

widget/headless/HeadlessWidget.h

@@ -106,6 +106,10 @@ public:
     // Headless widgets have no title, so just ignore it.
     return NS_OK;
   }
+  virtual nsresult SetNonClientMargins(LayoutDeviceIntMargin &margins) {
+    // Headless widgets have no chrome margins, so just ignore the call.
+    return NS_OK;
+  }
   virtual LayoutDeviceIntPoint WidgetToScreenOffset() override;
   virtual void SetInputContext(const InputContext& aContext,
                                const InputContextAction& aAction) override