ClearChildNeedsReattachLayoutTree after pseudo elements done.

Instead of doing this call inside RebuildChildrenLayoutTrees, wait until
we have finished all pseudo element children as well. This is done in
preparation for ::first-letter update changes and split out from [1].

The other call-site for RebuildChildrenLayoutTrees,
ShadowRoot::RebuildLayoutTree, already cleared this flag afterwards.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/1155591

Bug: 847218
Change-Id: I72816c5249bcb863fab906ffef90c026619d8165
Reviewed-on: https://chromium-review.googlesource.com/1156387Reviewed-by: Anders Ruud <andruud@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579806}

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

@@ -1445,7 +1445,6 @@ void ContainerNode::RebuildChildrenLayoutTrees(
   // This is done in ContainerNode::AttachLayoutTree but will never be cleared
   // if we don't enter ContainerNode::AttachLayoutTree so we do it here.
   ClearChildNeedsStyleRecalc();
-  ClearChildNeedsReattachLayoutTree();
 }
 
 void ContainerNode::CheckForSiblingStyleChanges(SiblingCheckType change_type,

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

@@ -2524,6 +2524,7 @@ void Element::RebuildLayoutTree(WhitespaceAttacher& whitespace_attacher) {
     RebuildPseudoElementLayoutTree(kPseudoIdBefore, *child_attacher);
     RebuildPseudoElementLayoutTree(kPseudoIdBackdrop, *child_attacher);
     RebuildPseudoElementLayoutTree(kPseudoIdFirstLetter, *child_attacher);
+    ClearChildNeedsReattachLayoutTree();
   }
   DCHECK(!NeedsStyleRecalc());
   DCHECK(!ChildNeedsStyleRecalc());