Run CSSOM smooth scroll animations on the compositor when possible

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

 // A SmoothScrollInterruptionTest verifies that in-progress smooth scrolls
 // stop when interrupted by an instant scroll, another smooth scroll, a
 // touch scroll, or a mouse wheel scroll.
 //
 // The only SmoothScrollInerruptionTest method that should be called by
 // outside code is run().
 //
 // Creates a SmoothScrollInterruptionTest with arguments:
 // scrollElement - Element being scrolled.
 // innerPoint - Absolute position (expressed as a dictionary with x and y fields)
@@ skipping 29 matching lines @@
         return;
     }
 
     var testCase = this.testCases[this.currentTestCase];
     this.asyncTest = async_test(testCase.description);
 
     var scrollElement = this.scrollElement;
     var scrollStartPoint = this.scrollStartPoint;
 
     scrollElement.scrollTop = scrollStartPoint;
+    window.requestAnimationFrame(this.performSmoothScroll.bind(this));
+}
+
+SmoothScrollInterruptionTest.prototype.performSmoothScroll = function() {
+    var testCase = this.testCases[this.currentTestCase];
+    var scrollElement = this.scrollElement;
+    var scrollStartPoint = this.scrollStartPoint;
+
     this.jsScroll(this.scrollEndPoint);
     this.asyncTest.step(function() {
         assert_equals(scrollElement.scrollTop, scrollStartPoint);
     });
 
     if (scrollElement.scrollTop == this.scrollEndPoint) {
         // We've instant-scrolled, and failed the assert above.
         this.testCaseComplete();
         return;
     }
@@ skipping 19 matching lines @@
 }
 
 SmoothScrollInterruptionTest.prototype.testCaseComplete = function() {
     this.asyncTest.done();
 
     this.currentTestCase++;
     this.startNextTestCase();
 }
 
 SmoothScrollInterruptionTest.prototype.run = function() {
-    setup({explicit_done: true});
+    setup({explicit_done: true, explicit_timeout: true});
     this.startNextTestCase();
 }
 
 SmoothScrollInterruptionTest.prototype.allTestCasesComplete = function() {
     done();
 }
 
 // A SmoothScrollInterruptionTestCase represents a single way of interrupting
 // a smooth scroll and verifying that the smooth scroll gets canceled.
 //
@@ skipping 81 matching lines @@
 }
 
 function interruptWithWheelScroll(smoothScrollTest) {
     if (window.eventSender) {
         window.eventSender.mouseMoveTo(smoothScrollTest.innerPoint.x, smoothScrollTest.innerPoint.y);
         window.eventSender.mouseScrollBy(0, -10);
     } else {
         document.write("This test does not work in manual mode.");
     }
 }