Don't invalidate z-order list when creating a ScrollableArea

This fixes performance regression caused by crrev.com/c/2547435 by
avoiding z-order list refresh when there will be no overlay scroll
controls. The invalidation is not necessary because we'll invalidate
z-order list when the ScrollableArea actually creates an overlay
scroll control.

TEST=Existing tests

Bug: 1149793, 1150795
Change-Id: Ifc380b891b9c9155b706c0d5c24c02685bf18a32
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2550586Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#829545}

third_party/blink/renderer/core/paint/paint_layer.cc

@@ -1610,8 +1610,8 @@ void PaintLayer::UpdateScrollableArea() {
   }
 
   GetLayoutObject().SetNeedsPaintPropertyUpdate();
-  // Need to update z-ordering of overlay overflow controls.
-  if (!scrollable_area_ || NeedsReorderOverlayOverflowControls())
+  // To clear z-ordering information of overlay overflow controls.
+  if (NeedsReorderOverlayOverflowControls())
     DirtyStackingContextZOrderLists();
   if (auto* compositor = Compositor())
     compositor->SetNeedsCompositingUpdate(kCompositingUpdateRebuildTree);