Update Element API for CSSOM smooth scrolling to match the spec

LayoutTests/fast/scroll-behavior/resources/scroll-behavior-test.js

 // A ScrollBehaviorTest runs a set of ScrollBehaviorTestCases. The only
 // ScrollBehaviorTest method that should be called by external code is run().
 
 // Creates a ScrollBehaviorTest with arguments:
 // scrollElement - Element being scrolled.
 // scrollEventTarget - Target for scroll events for |scrollElement|.
 // testsCases - Array of ScrollBehaviorTestCases.
 // getEndPosition - Callback that takes a test case and start position, and
 //                  returns the corresponding end position (where positions
 //                  are dictionaries with x and y fields).
@@ skipping 18 matching lines @@
 ScrollBehaviorTest.prototype.scrollListener = function(testCase) {
     if (testCase.waitForEnd) {
         if (this.scrollElement.scrollLeft == testCase.endX && this.scrollElement.scrollTop == testCase.endY)
             this.testCaseComplete();
         return;
     }
 
     // Wait for an intermediate frame, then instant-scroll to the end state.
     if ((this.scrollElement.scrollLeft != testCase.startX || this.scrollElement.scrollTop != testCase.startY) &&
         (this.scrollElement.scrollLeft != testCase.endX || this.scrollElement.scrollTop != testCase.endY)) {
-        this.scrollElement.scrollLeft = {x: testCase.endX, behavior: "instant"};
-        this.scrollElement.scrollTop = {y: testCase.endY, behavior: "instant"};
+        this.scrollElement.scrollTo({left: testCase.endX, top: testCase.endY, behavior: "instant"});
         this.testCaseComplete();
     }
 };
 
 ScrollBehaviorTest.prototype.startNextTestCase = function() {
     if (this.currentTestCase >= this.testCases.length) {
         this.allTestCasesComplete();
         return;
     }
     var testCase = this.testCases[this.currentTestCase];
@@ skipping 73 matching lines @@
 
 ScrollBehaviorTestCase.prototype.setStartPosition = function(startPosition) {
     this.startX = startPosition.x;
     this.startY = startPosition.y;
 }
 
 ScrollBehaviorTestCase.prototype.setEndPosition = function(endPosition) {
     this.endX = endPosition.x;
     this.endY = endPosition.y;
 }