Revert "Full AttachLayoutTree on a new Document not necessary."

This reverts commit 18a9ef45.

Reason for revert: Broke the accessibility test (again): https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Win10%20Tests%20x64/29232

Please note this does not look like a flake, after this landed a dozen runs failed consistently. 

Original change's description:
> Full AttachLayoutTree on a new Document not necessary.
> 
> We called AttachLayoutTree on a new Document which just got its
> LayoutView created. The new Document should not have any child nodes,
> it's not marked initially dirty, but trying to land this change the
> first time we caused flakiness for a Window accessibility test.
> 
> Instead of the AttachLayoutTree call, do the AXObjectCache update which
> would otherwise be called from Node::AttachLayoutTree.
> 
> This is a new attempt after the previous got reverted:
> 
> https://chromium-review.googlesource.com/c/chromium/src/+/1317889
> 
> Bug: 902050
> Change-Id: I4e366013549627b54f016d76b3dcbf12ea115f70
> Reviewed-on: https://chromium-review.googlesource.com/c/1319610
> Reviewed-by: Anders Ruud <andruud@chromium.org>
> Commit-Queue: Rune Lillesveen <futhark@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#605657}

TBR=futhark@chromium.org,andruud@chromium.org

Change-Id: Ied8ff9426940489ac4693abdb34001f44b13478c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 902050
Reviewed-on: https://chromium-review.googlesource.com/c/1321229Reviewed-by: Dmitry Titov <dimich@chromium.org>
Commit-Queue: Dmitry Titov <dimich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605839}

third_party/blink/renderer/core/dom/document.cc

@@ -2744,9 +2744,12 @@ void Document::Initialize() {
   layout_view_->Compositor()->SetNeedsCompositingUpdate(
       kCompositingUpdateAfterCompositingInputChange);
 
-  DCHECK(!firstChild());
-  if (AXObjectCache* cache = ExistingAXObjectCache())
-    cache->UpdateCacheAfterNodeIsAttached(this);
+  {
+    ReattachLegacyLayoutObjectList legacy_layout_objects(*this);
+    AttachContext context;
+    ContainerNode::AttachLayoutTree(context);
+    legacy_layout_objects.ForceLegacyLayoutIfNeeded();
+  }
 
   // The TextAutosizer can't update layout view info while the Document is
   // detached, so update now in case anything changed.