[LayoutNG] Enable final unit test for scroll anchoring.

It was disabled, because it used to fail. Now it passes. Don't remember
the details.

Bug: 889449
Change-Id: I0cd4b0f6fa5bb37494e814818740b3e35afc6fbe
Reviewed-on: https://chromium-review.googlesource.com/c/1333374Reviewed-by: Steve Kobes <skobes@chromium.org>
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607576}

third_party/blink/renderer/core/layout/scroll_anchor_test.cc

@@ -436,10 +436,6 @@ TEST_P(ScrollAnchorTest, FlexboxDelayedAdjustmentRespectsSANACLAP) {
 // TODO(skobes): Convert this to web-platform-tests when document.rootScroller
 // is launched (http://crbug.com/505516).
 TEST_P(ScrollAnchorTest, NonDefaultRootScroller) {
-  // TODO(crbug.com/889449): The test fails in LayoutNG mode.
-  if (RuntimeEnabledFeatures::LayoutNGEnabled())
-    return;
-
   SetBodyInnerHTML(R"HTML(
     <style>
         ::-webkit-scrollbar {