Move the CSS Device Adaptation @viewport rule support behind a runtime flag

Source/core/dom/Document.cpp

Index: Source/core/dom/Document.cpp
diff --git a/Source/core/dom/Document.cpp b/Source/core/dom/Document.cpp
index 063c4fd4ee7d7fc872fb604b541e39a950be54e9..647e85bf24061907daa638b1ddc197e3fb1c3918 100644
--- a/Source/core/dom/Document.cpp
+++ b/Source/core/dom/Document.cpp
@@ -5269,14 +5269,12 @@ IntSize Document::viewportSize() const
     return view()->visibleContentRect(ScrollableArea::IncludeScrollbars).size();
 }
 
-#if ENABLE(CSS_DEVICE_ADAPTATION)
 IntSize Document::initialViewportSize() const
 {
     if (!view())
         return IntSize();
-    return view()->initialViewportSize();
+    return view()->unscaledVisibleContentSize(ScrollableArea::IncludeScrollbars);
 }
-#endif
 
 Node* eventTargetNodeForDocument(Document* doc)
 {