Run CSSOM smooth scroll animations on the compositor when possible

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

Index: LayoutTests/fast/scroll-behavior/resources/scroll-interruption-test.js
diff --git a/LayoutTests/fast/scroll-behavior/resources/scroll-interruption-test.js b/LayoutTests/fast/scroll-behavior/resources/scroll-interruption-test.js
index c27dc9b30318d13fae3e97e0643de29a26c65e31..09fa4c9060d75387cfc4ae686ab210e00f69ddf7 100644
--- a/LayoutTests/fast/scroll-behavior/resources/scroll-interruption-test.js
+++ b/LayoutTests/fast/scroll-behavior/resources/scroll-interruption-test.js
@@ -47,6 +47,14 @@ SmoothScrollInterruptionTest.prototype.startNextTestCase = function() {
     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);
@@ -86,7 +94,7 @@ SmoothScrollInterruptionTest.prototype.testCaseComplete = function() {
 }
 
 SmoothScrollInterruptionTest.prototype.run = function() {
-    setup({explicit_done: true});
+    setup({explicit_done: true, explicit_timeout: true});
     this.startNextTestCase();
 }