Implement Document.scrollingElement API behind experimental feature flag

LayoutTests/fast/dom/Element/scrollTop-scrollLeft-body-expected.txt

Index: LayoutTests/fast/dom/Element/scrollTop-scrollLeft-body-expected.txt
diff --git a/LayoutTests/fast/dom/Element/scrollTop-scrollLeft-body-expected.txt b/LayoutTests/fast/dom/Element/scrollTop-scrollLeft-body-expected.txt
index 2d6e3f09ad8b19380bc382d9abdef0f164b79a25..9d93078520ab1ec166cb968930445da5ef841dca 100644
--- a/LayoutTests/fast/dom/Element/scrollTop-scrollLeft-body-expected.txt
+++ b/LayoutTests/fast/dom/Element/scrollTop-scrollLeft-body-expected.txt
@@ -1,49 +1,57 @@
-Test for scrollTop/Left values of scrollable document.body in quirks and strict modes
+Test for scrollTop/Left and scrollingElement values of scrollable document.body in quirks and strict modes
 
 On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
 
 
+PASS iframeStrict1.contentDocument.scrollingElement is iframeStrict1.contentDocument.documentElement
 PASS iframeStrict1.contentDocument.documentElement.scrollTop is 0
 PASS iframeStrict1.contentDocument.documentElement.scrollLeft is 100
 PASS iframeStrict1.contentDocument.body.scrollTop is 200
 PASS iframeStrict1.contentDocument.body.scrollLeft is 0
 PASS iframeStrict1.contentWindow.pageYOffset is 0
 PASS iframeStrict1.contentWindow.pageXOffset is 100
+PASS iframeQuirks1.contentDocument.scrollingElement is null
 PASS iframeQuirks1.contentDocument.documentElement.scrollTop is 0
 PASS iframeQuirks1.contentDocument.documentElement.scrollLeft is 0
 PASS iframeQuirks1.contentDocument.body.scrollTop is 200
 PASS iframeQuirks1.contentDocument.body.scrollLeft is 0
 PASS iframeQuirks1.contentWindow.pageYOffset is 0
 PASS iframeQuirks1.contentWindow.pageXOffset is 0
+PASS iframeStrict2.contentDocument.scrollingElement is iframeStrict2.contentDocument.documentElement
 PASS iframeStrict2.contentDocument.documentElement.scrollTop is 0
 PASS iframeStrict2.contentDocument.documentElement.scrollLeft is 100
 PASS iframeStrict2.contentDocument.body.scrollTop is 0
 PASS iframeStrict2.contentDocument.body.scrollLeft is 0
 PASS iframeStrict2.contentWindow.pageYOffset is 0
 PASS iframeStrict2.contentWindow.pageXOffset is 100
+PASS iframeQuirks2.contentDocument.scrollingElement is null
 PASS iframeQuirks2.contentDocument.documentElement.scrollTop is 0
 PASS iframeQuirks2.contentDocument.documentElement.scrollLeft is 0
 PASS iframeQuirks2.contentDocument.body.scrollTop is 0
 PASS iframeQuirks2.contentDocument.body.scrollLeft is 0
 PASS iframeQuirks2.contentWindow.pageYOffset is 0
 PASS iframeQuirks2.contentWindow.pageXOffset is 0
+PASS iframeStrict3.contentDocument.scrollingElement is iframeStrict3.contentDocument.documentElement
 PASS iframeStrict3.contentDocument.documentElement.scrollTop is 0
 PASS iframeStrict3.contentDocument.documentElement.scrollLeft is 100
 PASS iframeStrict3.contentDocument.body.scrollTop is 200
 PASS iframeStrict3.contentDocument.body.scrollLeft is 0
 PASS iframeStrict3.contentWindow.pageYOffset is 0
 PASS iframeStrict3.contentWindow.pageXOffset is 100
+PASS iframeQuirks3.contentDocument.scrollingElement is null
 PASS iframeQuirks3.contentDocument.documentElement.scrollTop is 0
 PASS iframeQuirks3.contentDocument.documentElement.scrollLeft is 0
 PASS iframeQuirks3.contentDocument.body.scrollTop is 200
 PASS iframeQuirks3.contentDocument.body.scrollLeft is 0
 PASS iframeQuirks3.contentWindow.pageYOffset is 0
 PASS iframeQuirks3.contentWindow.pageXOffset is 0
+PASS iframeStrict4.contentDocument.scrollingElement is iframeStrict4.contentDocument.documentElement
 PASS iframeStrict4.contentDocument.body.scrollTop is 0
 PASS iframeStrict4.contentDocument.body.scrollLeft is 0
-PASS iframeQuirks4.contentDocument.body.scrollTop is 0
-PASS iframeQuirks4.contentDocument.body.scrollLeft is 0
+PASS iframeQuirks4.contentDocument.scrollingElement is iframeQuirks4.contentDocument.body
+PASS iframeQuirks4.contentDocument.body.scrollTop is 4000
+PASS iframeQuirks4.contentDocument.body.scrollLeft is 3000
 PASS successfullyParsed is true
 
 TEST COMPLETE
-           
+