Fix flake in threaded CSSOM smooth scroll layout tests

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).
 // jsScroll - Callback that takes a test case and executes the corresponding
 //            js-driven scroll (e.g. by setting scrollLeft/scrollTop or by
 //            calling scroll, scrollTo, or scrollBy). This should assume that
 //            scrollElement's scroll-behavior CSS property has already been
 //            set appropriately.
 function ScrollBehaviorTest(scrollElement,
                             scrollEventTarget,
                             testCases,
                             getEndPosition,
                             jsScroll) {
     this.scrollElement = scrollElement;
     this.scrollEventTarget = scrollEventTarget;
     this.testCases = testCases;
     this.currentTestCase = 0;
     this.getEndPosition = getEndPosition;
     this.jsScroll = jsScroll;
 }
 
 ScrollBehaviorTest.prototype.scrollListener = function(testCase) {
-    if (testCase.waitForEnd) {
-        if (this.scrollElement.scrollLeft == testCase.endX && this.scrollElement.scrollTop == testCase.endY)
-            this.testCaseComplete();
+    var endReached = (this.scrollElement.scrollLeft == testCase.endX && this.scrollElement.scrollTop == testCase.endY);
+    if (endReached) {
+        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)) {
+    if (testCase.waitForEnd)
+        return;
+
+    // Wait for the animation to start, then instant-scroll to the end state.
+    if (this.scrollElement.scrollLeft != testCase.startX || this.scrollElement.scrollTop != testCase.startY) {
         // 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"});
         testCase.waitForEnd = true;
     }
 };
 
@@ skipping 79 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;
 }