Exclude position:absolute elements when picking an anchor for ScrollAnchoring

third_party/WebKit/LayoutTests/fast/layout/scroll-anchoring/resources/scroll-anchoring-test.js

+// Contains helper methods to verify expectations for tests related to Scroll
+// Anchoring
+var frame = () => new Promise((resolve) => { requestAnimationFrame(resolve); });
+
+var waitFor = function(condition, failmsg, deadline) {
+    if (!deadline) deadline = Date.now() + 1000;
+    if (condition()) return Promise.resolve();
+    else if (Date.now() > deadline) return Promise.reject(failmsg);
+    else return frame().then(() => waitFor(condition, failmsg, deadline));
+};
+
+var waitFrames = function(n, condition, failmsg) {
+    var p = Promise.resolve();
+    var check = () => (!condition || condition() ?
+        Promise.resolve() : Promise.reject(failmsg));
+    while (n--)
+        p = p.then(check).then(frame);
+    return p.then(check);
+};
+
+var scrollSettlesAt = function(expectedY) {
+    return waitFor(() => (scrollY == expectedY),
+            "scroll did not reach " + expectedY)
+        .then(() => waitFrames(3))
+        .then(() => waitFrames(3, () => (scrollY == expectedY),
+            "scroll did not stay at " + expectedY));
+};