Handle nested position:sticky elements correctly (main thread)

third_party/WebKit/LayoutTests/fast/css/sticky/nested/sticky-nested-inline.html

+<style>
+body {
+  margin: 0;
+}
+
+.scroller {
+  overflow: hidden; /* hide scrollbars */
+  width: 200px;
+  height: 350px;
+  outline: 2px solid black;
+}
+
+.container {
+  width: 200px;
+  height: 700px;
+}
+
+.outerSticky {
+  display: inline;
+  position: sticky;
+  top: 0;
+}
+
+.innerSticky {
+  display: inline;
+  position: sticky;
+  top: 25px;
+}
+</style>
+
+<script>
+if (window.testRunner)
+  testRunner.waitUntilDone();
+
+function finishTest() {
+  document.querySelector('.scroller').scrollTop = 200;
+  if (window.testRunner)
+    testRunner.notifyDone();
+}
+
+window.addEventListener('load', function() {
+  requestAnimationFrame(function() {
+    requestAnimationFrame(finishTest);
+  });
+});
+</script>
+
+<div class="scroller">
+  <div class="container">
+    <div class="outerSticky">1<div class="innerSticky">2</div></div>
+  </div>
+</div>