Increase reliablity of scorllingcoordinator layout tests

LayoutTests/scrollingcoordinator/resources/non-fast-scrollable-region-testing.js

 // setMockScrollbarsEnabled doesn't actually invalidate scrollbars
 // so if we don't set it immediately, they won't repaint/relayout
 // correctly!  http://crbug.com/365509
 if (window.internals)
     window.internals.settings.setMockScrollbarsEnabled(true);
 
-function runNonFastScrollableRegionTest(scale) {
-    var invScale;
-    if (scale != undefined) {
-        if (window.eventSender)
-            eventSender.setPageScaleFactor(scale, 0, 0);
-        // FIXME: This is a hack for applyPageScaleFactorInCompositor() == false.
-        invScale = 1 / scale;
-    } else {
-        invScale = 1;
-    }
-
+// Draws green overlays for non-fast scrollable regions. This provides a visual
+// feedback that is useful when running the test interactively.
+function drawNonFastScrollableRegionOverlays() {
     var overlay = document.createElement("div");
     overlay.style.position = "absolute";
     overlay.style.left = 0;
     overlay.style.top = 0;
     overlay.style.opacity = 0.5;
 
     var rects = window.internals.nonFastScrollableRects(document);
     for (var i = 0; i < rects.length; i++) {
         var rect = rects[i];
         var patch = document.createElement("div");
         patch.style.position = "absolute";
         patch.style.backgroundColor = "#00ff00";
-        patch.style.left = rect.left * invScale + "px";
-        patch.style.top = rect.top * invScale + "px";
-        patch.style.width = rect.width * invScale + "px";
-        patch.style.height = rect.height * invScale + "px";
+        patch.style.left = rect.left + "px";
+        patch.style.top = rect.top  + "px";
+        patch.style.width = rect.width + "px";
+        patch.style.height = rect.height + "px";
 
         overlay.appendChild(patch);
     }
 
     document.body.appendChild(overlay);
 }
 
+// Waits for one RAF call to ensure compositing update has occurred and invokes task.
+function awaitCompsitingUpdate(task) {
+    window.requestAnimationFrame(task);
+}
+
+function rectToString(rect) {
+    return '[' + [rect.left, rect.top, rect.width, rect.height].join(', ') + ']';
+}
+