Clear layout_tree_rebuild_root_ on node removal.

There was a copy-paste error which cleared the invalidation root twice
instead. Since both marking and doing the actual rebuild of the layout
tree happens inside UpdateStyle(), we should never be in the situation
where we have a rebuild root when dom changes happen, so the clearing
should strictly not be necessary. Add a DCHECK to detect if this ever
happens.

Bug: 868810
Change-Id: Ibcc9ec21e7a094295833ee442bd723000013b01e
Reviewed-on: https://chromium-review.googlesource.com/1230040Reviewed-by: Anders Ruud <andruud@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592013}

third_party/blink/renderer/core/css/style_engine.cc

@@ -1512,7 +1512,8 @@ void StyleEngine::ChildrenRemoved(ContainerNode& parent) {
   }
   style_invalidation_root_.ChildrenRemoved(parent);
   style_recalc_root_.ChildrenRemoved(parent);
-  style_invalidation_root_.ChildrenRemoved(parent);
+  DCHECK(!layout_tree_rebuild_root_.GetRootNode());
+  layout_tree_rebuild_root_.ChildrenRemoved(parent);
 }
 
 void StyleEngine::CollectMatchingUserRules(