[compositor-worker] root scrolling layer is associated with document scrolling element

third_party/WebKit/LayoutTests/virtual/threaded/fast/compositorworker/resources/root-scroller.js

Index: third_party/WebKit/LayoutTests/virtual/threaded/fast/compositorworker/resources/root-scroller.js
diff --git a/third_party/WebKit/LayoutTests/virtual/threaded/fast/compositorworker/resources/root-scroller.js b/third_party/WebKit/LayoutTests/virtual/threaded/fast/compositorworker/resources/root-scroller.js
new file mode 100644
index 0000000000000000000000000000000000000000..0caf2ba09917b39740317b7a3c39a8b0655929eb
--- /dev/null
+++ b/third_party/WebKit/LayoutTests/virtual/threaded/fast/compositorworker/resources/root-scroller.js
@@ -0,0 +1,19 @@
+self.onmessage = function(msg) {
+  awaitProxyInit(msg.data.proxy).then((proxy) => {
+    proxy.scrollLeft = 500;
+    proxy.scrollTop = 500;
+    postMessage({});
+  });
+}
+
+function awaitProxyInit(proxy) {
+  return new Promise((resolve, reject) => {
+    function check() {
+      if (proxy.initialized)
+        resolve(proxy);
+      else
+        requestAnimationFrame(check);
+    }
+    requestAnimationFrame(check);
+  });
+}