Enabled using viewport on desktop browsers behind experimental flag

Source/core/page/FrameView.h

Index: Source/core/page/FrameView.h
diff --git a/Source/core/page/FrameView.h b/Source/core/page/FrameView.h
index ff021cf007f9659f1daf79d2159ae7c9807b55b5..1c96bb3156bc1baf21a32cbec278284c8eb7db90 100644
--- a/Source/core/page/FrameView.h
+++ b/Source/core/page/FrameView.h
@@ -86,6 +86,9 @@ public:
     void setMarginWidth(LayoutUnit);
     void setMarginHeight(LayoutUnit);
 
+    virtual IntSize layoutSize(VisibleContentRectIncludesScrollbars = ExcludeScrollbars) const;
+    void setLayoutSize(const IntSize&);
+
     virtual void setCanHaveScrollbars(bool);
     void updateCanHaveScrollbars();
 
@@ -464,6 +467,9 @@ private:
     IntSize m_lastViewportSize;
     float m_lastZoomFactor;
 
+    // Client can explicitly set a layout size, if unset the frameRect is used
+    IntSize m_layoutSize;
+
     AtomicString m_mediaType;
     AtomicString m_mediaTypeWhenNotPrinting;