Fix timeouts in overflow-scroll-animates.html with threaded compositing.

LayoutTests/fast/scroll-behavior/overflow-scroll-animates.html

Index: LayoutTests/fast/scroll-behavior/overflow-scroll-animates.html
diff --git a/LayoutTests/fast/scroll-behavior/overflow-scroll-animates.html b/LayoutTests/fast/scroll-behavior/overflow-scroll-animates.html
index a88a2b79f3f4605999b213d809ac23d3fd2862fc..3d960ed772e753e8e181b489d2a5d05e72a2d64b 100644
--- a/LayoutTests/fast/scroll-behavior/overflow-scroll-animates.html
+++ b/LayoutTests/fast/scroll-behavior/overflow-scroll-animates.html
@@ -8,8 +8,8 @@
 }
 
 #content {
-  width: 7500px;
-  height: 7500px;
+  width: 750px;
+  height: 750px;
   background-color: blue;
 }
 
@@ -39,16 +39,19 @@ function nextConfig() {
 
     element.addEventListener("scroll", onElementScroll);
 
-    eventSender.mouseMoveTo(100, 100);
-    eventSender.mouseScrollBy(0, -2, /* paged */ false,
-        config.preciseDeltas);
-    scrollStart = performance.now();
+    testRunner.displayAsyncThen(function() {
+        eventSender.mouseMoveTo(100, 100);
+        eventSender.mouseScrollBy(0, -2, /* paged */ false,
+            config.preciseDeltas);
+        scrollStart = performance.now();
+    });
 }
 
 function reset() {
     element.removeEventListener("scroll", onElementScroll);
     element.scrollTop = 0;
     waitingForScroll = false;
+    scrollStart = undefined;
 }
 
 function onElementScroll() {
@@ -59,6 +62,8 @@ function onElementScroll() {
             reset();
             nextConfig();
         }
+    } else if (!scrollStart) {
+        return;
     } else if (config.expectSmooth) {
         shouldNotBe("element.scrollTop", "80");
         waitingForScroll = true;