Update layout tests to work when smooth scrolling is enabled by default

third_party/WebKit/LayoutTests/fast/scrolling/fixed-position-behind-scrollbar.html

Index: third_party/WebKit/LayoutTests/fast/scrolling/fixed-position-behind-scrollbar.html
diff --git a/third_party/WebKit/LayoutTests/fast/scrolling/fixed-position-behind-scrollbar.html b/third_party/WebKit/LayoutTests/fast/scrolling/fixed-position-behind-scrollbar.html
index df860f23c72deaa91ac75f55987b2bbb25204191..a6c3a2203d610da09ae62ddc84a01c718871ab8d 100644
--- a/third_party/WebKit/LayoutTests/fast/scrolling/fixed-position-behind-scrollbar.html
+++ b/third_party/WebKit/LayoutTests/fast/scrolling/fixed-position-behind-scrollbar.html
@@ -19,14 +19,21 @@ element, and that the scrollbars are excluded from the viewport for determining
 the element's position.
 <div id="f"></div>
 <script>
+window.jsTestIsAsync = true;
 
-eventSender.mouseMoveTo(790, 500);
-eventSender.mouseDown();
-eventSender.mouseUp();
+function finishTest() {
+  var rect = document.querySelector("#f").getBoundingClientRect();
+  shouldBe(rect.left + "," + rect.top, "205,205");
+  finishJSTest();
+}
 
-var rect = document.querySelector("#f").getBoundingClientRect();
+function runTest() {
+  eventSender.mouseMoveTo(790, 500);
+  eventSender.mouseDown();
+  eventSender.mouseUp();
+  shouldBecomeEqual('scrollY > 0', 'true', finishTest);
+}
 
-shouldBeTrue(String(scrollY > 0));
-shouldBe(rect.left + "," + rect.top, "205,205");
+onload = runTest;
 
 </script>