Implement Document.scrollingElement API behind experimental feature flag

Source/core/html/HTMLBodyElement.h

Index: Source/core/html/HTMLBodyElement.h
diff --git a/Source/core/html/HTMLBodyElement.h b/Source/core/html/HTMLBodyElement.h
index 6537422c9866d7dda859035eee13d42eaab25df8..5938d796acbbfbfc5e4c0039b671863b0c4be46a 100644
--- a/Source/core/html/HTMLBodyElement.h
+++ b/Source/core/html/HTMLBodyElement.h
@@ -60,18 +60,6 @@ private:
     virtual const QualifiedName& subResourceAttributeName() const override;
 
     virtual bool supportsFocus() const override;
-
-    virtual double scrollLeft() override;
-    virtual void setScrollLeft(double) override;
-
-    virtual double scrollTop() override;
-    virtual void setScrollTop(double) override;
-
-    virtual int scrollHeight() override;
-    virtual int scrollWidth() override;
-
-    virtual void scrollBy(const ScrollToOptions&) override;
-    virtual void scrollTo(const ScrollToOptions&) override;
 };
 
 } // namespace blink