Implement Document.scrollingElement API behind experimental feature 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 2db28fbdbfe5ed72ccc9c380b25e3289229df80d..7223c5e234d13944a9798e9e98419a167d2268ef 100644
--- a/Source/core/dom/Document.cpp
+++ b/Source/core/dom/Document.cpp
@@ -1266,6 +1266,23 @@ PassRefPtrWillBeRawPtr<Range> Document::caretRangeFromPoint(int x, int y)
     return Range::createAdjustedToTreeScope(*this, rangeCompliantPosition);
 }
 
+Element* Document::scrollingElement() const
+{
+    if (RuntimeEnabledFeatures::scrollTopLeftInteropEnabled()) {
+        if (inQuirksMode()) {
+            // TODO(rbyers): This detail is under debate on www-style. crbug.com/475531
+            if (body()->layoutBox() && body()->layoutBox()->hasOverflowClip())
+                return nullptr;
+
+            return body();
+        }
+
+        return documentElement();
+    }
+
+    return body();
+}
+
 /*
  * Performs three operations:
  *  1. Convert control characters to spaces