Update Element API for CSSOM smooth scrolling to match the spec

LayoutTests/fast/scroll-behavior/overflow-hidden-scrollTop.html

Index: LayoutTests/fast/scroll-behavior/overflow-hidden-scrollTop.html
diff --git a/LayoutTests/fast/scroll-behavior/overflow-hidden-scrollTop.html b/LayoutTests/fast/scroll-behavior/overflow-hidden-scrollTop.html
index 5e9b83fec0d203e51257d14e7527211a78fbe1ff..9d1a46f4541de4e72a29eff13528dde0cbfaa240 100644
--- a/LayoutTests/fast/scroll-behavior/overflow-hidden-scrollTop.html
+++ b/LayoutTests/fast/scroll-behavior/overflow-hidden-scrollTop.html
@@ -27,26 +27,16 @@
 
     function jsScroll(testCase) {
       var element = document.getElementById("container");
-      if (testCase.js) {
-        element.scrollTop = {y: testCase.y, behavior: testCase.js};
-      } else {
-        element.scrollTop = testCase.y;
-      }
+      element.scrollTop = testCase.y;
     }
 
     const testScrolls = [
-      {js: "instant", css: "auto", x: 0, y: 2},
-      {js: "instant", css: "smooth", x: 0, y: 3},
-      {js: "auto", css: "auto", x: 0, y: 4},
-      {js: "", css: "auto", x: 0, y: 5},
-      {js: "smooth", css: "auto", waitForEnd: true, x: 0, y: 15},
-      {js: "smooth", css: "smooth", waitForEnd: true, x: 0, y: 25},
-      {js: "auto", css: "smooth", waitForEnd: true, x: 0, y: 35},
-      {js: "", css: "smooth", waitForEnd: true, x: 0, y: 45},
-      {js: "smooth", css: "auto", waitForEnd: false, x: 0, y: 4100},
-      {js: "smooth", css: "smooth", waitForEnd: false, x: 0, y: 20},
-      {js: "auto", css: "smooth", waitForEnd: false, x: 0, y: 4000},
-      {js: "", css: "smooth", waitForEnd: false, x: 0, y: 5},
+      {css: "auto", x: 0, y: 2},
+      {css: "auto", x: 0, y: 4},
+      {css: "smooth", waitForEnd: true, x: 0, y: 25},
+      {css: "smooth", waitForEnd: true, x: 0, y: 45},
+      {css: "smooth", waitForEnd: false, x: 0, y: 4100},
+      {css: "smooth", waitForEnd: false, x: 0, y: 20},
     ];
 
     function doTest()