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This logic doesn't make a difference in any tests.
Notably: continuation.html in web tests passes.

This CL progresses towards removal of
ShouldUseLayoutBuilderTraversal(), simplifying AddChildren() and
removing conditions that lead to lack of symmetry in
child-parent relationships.

Bug: None
Change-Id: Ic436c4dd51ba7d12cd6adec0f7d35cdbab7dc487
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2563863Reviewed-by: Alice Boxhall <aboxhall@chromium.org>
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#832024}