Correct for enclosing layers scroll position in sticky_data->main_thread_offset

third_party/WebKit/LayoutTests/compositing/overflow/composited-sticky-element-enclosing-layers.html

Index: third_party/WebKit/LayoutTests/compositing/overflow/composited-sticky-element-enclosing-layers.html
diff --git a/third_party/WebKit/LayoutTests/compositing/overflow/composited-sticky-element-enclosing-layers.html b/third_party/WebKit/LayoutTests/compositing/overflow/composited-sticky-element-enclosing-layers.html
new file mode 100644
index 0000000000000000000000000000000000000000..6985f7d4024cc373a4c99c2137c49a24cfaf4ece
--- /dev/null
+++ b/third_party/WebKit/LayoutTests/compositing/overflow/composited-sticky-element-enclosing-layers.html
@@ -0,0 +1,37 @@
+<script>
+onload = function() {
+  scroller.scrollTop = 200;
+}
+</script>
+<style>
+.composited {
+  will-change: transform;
+}
+
+#scroller {
+  overflow: scroll;
+  height: 200px;
+  width: 200px;
+}
+
+.container {
+  height: 500px;
+  background: red;
+}
+
+.sticky {
+  position: sticky;
+  top: 25px;
+  height: 50px;
+  background: green;
+}
+</style>
+<div id="scroller" class="composited">
+  <div class="composited container">
+    <div class="composited container">
+      <div class="composited container">

[chrishtr, 2017/02/28 19:36:28]

Why have three nested "composited container" elements?

[smcgruer, 2017/03/01 19:58:39]

No real reason. Makes me feel better that it still works correctly in the
presence of multiple intermediate layers, but only one is required. Removed the
rest.

+        <div class="composited sticky"></div>
+      </div>
+    </div>
+  </div>
+</div>