Run CSSOM smooth scroll animations on the compositor when possible

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)) {
+        // Instant scroll, and then wait for the next scroll event. This allows
+        // the instant scroll to propagate to the compositor (when using
+        // composited scrolling) so that the next smooth scroll starts at this
+        // position (the compositor always starts smooth scrolls at the current
+        // scroll position on the compositor thread).
         this.scrollElement.scrollTo({left: testCase.endX, top: testCase.endY, behavior: "instant"});
-        this.testCaseComplete();
+        testCase.waitForEnd = true;
     }
 };
 
 ScrollBehaviorTest.prototype.startNextTestCase = function() {
     if (this.currentTestCase >= this.testCases.length) {
         this.allTestCasesComplete();
         return;
     }
     var testCase = this.testCases[this.currentTestCase];
     var isSmoothTest = (testCase.js == "smooth" || (testCase.css == "smooth" && testCase.js != "instant"));
@@ skipping 37 matching lines @@
     if (currentScrollListener) {
         this.scrollEventTarget.removeEventListener("scroll", currentScrollListener);
     }
     this.asyncTest.done();
 
     this.currentTestCase++;
     this.startNextTestCase();
 }
 
 ScrollBehaviorTest.prototype.run = function() {
-    setup({explicit_done: true});
+    setup({explicit_done: true, explicit_timeout: true});
     this.startNextTestCase();
 }
 
 ScrollBehaviorTest.prototype.allTestCasesComplete = function() {
     done();
 }
 
 
 // A ScrollBehaviorTestCase represents a single scroll.
 //
@@ skipping 14 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;
 }