Guard the rework of documentElement|body.scrollTop/Left under a runtime flag

Source/core/dom/Element.cpp

Index: Source/core/dom/Element.cpp
diff --git a/Source/core/dom/Element.cpp b/Source/core/dom/Element.cpp
index 01a776123369040ba8f456d07d240bc9561a6aba..0fb87fd82ba96ed9c6922149e4532f81f10bc1bc 100644
--- a/Source/core/dom/Element.cpp
+++ b/Source/core/dom/Element.cpp
@@ -677,7 +677,13 @@ int Element::scrollLeft()
 {
     document().updateLayoutIgnorePendingStylesheets();
 
-    if (document().documentElement() == this) {
+    if (document().documentElement() != this) {
+        if (RenderBox* rend = renderBox())
+            return adjustForAbsoluteZoom(rend->scrollLeft(), rend);
+        return 0;
+    }
+
+    if (RuntimeEnabledFeatures::scrollTopLeftInteropEnabled()) {
         if (document().inQuirksMode())
             return 0;
 
@@ -687,8 +693,6 @@ int Element::scrollLeft()
         }
     }
 
-    if (RenderBox* rend = renderBox())
-        return adjustForAbsoluteZoom(rend->scrollLeft(), rend);
     return 0;
 }
 
@@ -696,7 +700,13 @@ int Element::scrollTop()
 {
     document().updateLayoutIgnorePendingStylesheets();
 
-    if (document().documentElement() == this) {
+    if (document().documentElement() != this) {
+        if (RenderBox* rend = renderBox())
+            return adjustForAbsoluteZoom(rend->scrollTop(), rend);
+        return 0;
+    }
+
+    if (RuntimeEnabledFeatures::scrollTopLeftInteropEnabled()) {
         if (document().inQuirksMode())
             return 0;
 
@@ -706,8 +716,6 @@ int Element::scrollTop()
         }
     }
 
-    if (RenderBox* rend = renderBox())
-        return adjustForAbsoluteZoom(rend->scrollTop(), rend);
     return 0;
 }
 
@@ -715,7 +723,13 @@ void Element::setScrollLeft(int newLeft)
 {
     document().updateLayoutIgnorePendingStylesheets();
 
-    if (document().documentElement() == this) {
+    if (document().documentElement() != this) {
+        if (RenderBox* rend = renderBox())
+            rend->setScrollLeft(static_cast<int>(newLeft * rend->style()->effectiveZoom()));
+        return;
+    }
+
+    if (RuntimeEnabledFeatures::scrollTopLeftInteropEnabled()) {
         if (document().inQuirksMode())
             return;
 
@@ -726,24 +740,21 @@ void Element::setScrollLeft(int newLeft)
         if (!view)
             return;
 
-        // WHATWG spec says [1]: "If the element is the root element invoke scroll()
-        // with x as first argument and zero as second". Blink intentionally matches
-        // other engine's behaviors here, instead, where the 'y' scroll position is
-        // preversed. See [2].
-        // [1] http://dev.w3.org/csswg/cssom-view/#dom-element-scrollleft
-        // [2] https://www.w3.org/Bugs/Public/show_bug.cgi?id=23448
         view->setScrollPosition(IntPoint(static_cast<int>(newLeft * frame->pageZoomFactor()), view->scrollY()));
     }
-
-    if (RenderBox* rend = renderBox())
-        rend->setScrollLeft(static_cast<int>(newLeft * rend->style()->effectiveZoom()));
 }
 
 void Element::setScrollTop(int newTop)
 {
     document().updateLayoutIgnorePendingStylesheets();
 
-    if (document().documentElement() == this) {
+    if (document().documentElement() != this) {
+        if (RenderBox* rend = renderBox())
+            rend->setScrollTop(static_cast<int>(newTop * rend->style()->effectiveZoom()));
+        return;
+    }
+
+    if (RuntimeEnabledFeatures::scrollTopLeftInteropEnabled()) {
         if (document().inQuirksMode())
             return;
 
@@ -754,17 +765,8 @@ void Element::setScrollTop(int newTop)
         if (!view)
             return;
 
-        // WHATWG spec says [1]: "If the element is the root element invoke scroll()
-        // with zero as first argument and y as second". Blink intentionally
-        // matches other engine's behaviors here, instead, where the 'x' scroll
-        // position is preversed. See [2].
-        // [1] http://dev.w3.org/csswg/cssom-view/#dom-element-scrolltop
-        // [2] https://www.w3.org/Bugs/Public/show_bug.cgi?id=23448
         view->setScrollPosition(IntPoint(view->scrollX(), static_cast<int>(newTop * frame->pageZoomFactor())));
     }
-
-    if (RenderBox* rend = renderBox())
-        rend->setScrollTop(static_cast<int>(newTop * rend->style()->effectiveZoom()));
 }
 
 int Element::scrollWidth()