CC AnimationHost: Fix ProgrammaticScrollAnimator detach scope.

third_party/WebKit/LayoutTests/virtual/threaded/fast/scroll-behavior/subframe-scrollTo-close.html

+<!DOCTYPE html>
+<html>
+<head>
+<style>
+  #subframe {
+    width: 600px;
+    height: 600px;
+  }
+</style>
+<script src="../../../../resources/testharness.js"></script>
+<script src="../../../../resources/testharnessreport.js"></script>
+<script type="text/javascript">
+var asyncHandle = async_test("Test that closing window with ongoing scrollTo animation doesn't crash");
+
+if (window.testRunner)
+  testRunner.setCanOpenWindows();
+
+var wnd = window.open("../../../../fast/scroll-behavior/resources/subframe-with-scroll.html");
+
+function frameLoaded() {
+  window.requestAnimationFrame(() => {
+    wnd.close();
+    window.requestAnimationFrame(() => {
+      asyncHandle.done();
+    });
+  });
+}
+
+wnd.addEventListener('load', frameLoaded, false);
+</script>
+</head>
+</html>