Update Element API for CSSOM smooth scrolling to match the spec

LayoutTests/fast/scroll-behavior/overflow-scroll-scrollLeft.html

Index: LayoutTests/fast/scroll-behavior/overflow-scroll-scrollLeft.html
diff --git a/LayoutTests/fast/scroll-behavior/overflow-scroll-scrollLeft.html b/LayoutTests/fast/scroll-behavior/overflow-scroll-scrollLeft.html
index 174b917918356f2f01f1d23dcad0a6ba14e21b3e..6292a591f85c3b358350fbbd719e9878ebe91603 100644
--- a/LayoutTests/fast/scroll-behavior/overflow-scroll-scrollLeft.html
+++ b/LayoutTests/fast/scroll-behavior/overflow-scroll-scrollLeft.html
@@ -27,26 +27,16 @@
 
     function jsScroll(testCase) {
       var element = document.getElementById("container");
-      if (testCase.js) {
-        element.scrollLeft = {x: testCase.x, behavior: testCase.js};
-      } else {
-        element.scrollLeft = testCase.x;
-      }
+      element.scrollLeft = testCase.x;
     }
 
     const testScrolls = [
-      {js: "instant", css: "auto", x: 1, y: 0},
-      {js: "instant", css: "smooth", x: 2, y: 0},
-      {js: "auto", css: "auto", x: 3, y: 0},
-      {js: "", css: "auto", x: 4, y: 0},
-      {js: "smooth", css: "auto", waitForEnd: true, x: 10, y: 0},
-      {js: "smooth", css: "smooth", waitForEnd: true, x: 20, y: 0},
-      {js: "auto", css: "smooth", waitForEnd: true, x: 30, y: 0},
-      {js: "", css: "smooth", waitForEnd: true, x: 40, y: 0},
-      {js: "smooth", css: "auto", waitForEnd: false, x: 4000, y: 0},
-      {js: "smooth", css: "smooth", waitForEnd: false, x: 15, y: 0},
-      {js: "auto", css: "smooth", waitForEnd: false, x: 4100, y: 0},
-      {js: "", css: "smooth", waitForEnd: false, x: 10, y: 0},
+      {css: "auto", x: 1, y: 0},
+      {css: "auto", x: 4, y: 0},
+      {css: "smooth", waitForEnd: true, x: 20, y: 0},
+      {css: "smooth", waitForEnd: true, x: 40, y: 0},
+      {css: "smooth", waitForEnd: false, x: 4000, y: 0},
+      {css: "smooth", waitForEnd: false, x: 10, y: 0},
     ];
 
     function doTest()