Introduce window.internals.selected{Html,Text}ForClipboard()

Source/core/testing/Internals.h

Index: Source/core/testing/Internals.h
diff --git a/Source/core/testing/Internals.h b/Source/core/testing/Internals.h
index 028aef0aee82248560015a7406360affae8bb466..551f0d650676540474c4bf821c8532e05fc55a3f 100644
--- a/Source/core/testing/Internals.h
+++ b/Source/core/testing/Internals.h
@@ -341,6 +341,9 @@ public:
     //       Normal LayoutTests should use gc() instead as it would trigger both Blink GC and V8 GC.
     void forceBlinkGCWithoutV8GC();
 
+    String selectedHTMLForClipboard();
+    String selectedTextForClipboard();
+
 private:
     explicit Internals(Document*);
     Document* contextDocument() const;