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We don't know if there will be any slots added for an attached shadow
root. If no slots are added, there will be no slot re-assignment and no
other way to inform the shadow host children that their flat tree
position changed.

LazyReattachLayoutTree will not clear ComputedStyle because it uses true
for performing_reattach in AttachContext. Instead call
RemovedFromFlatTree which detaches the layout tree and clears the
computed style.

Bug: 932871
Change-Id: I3dd8f7f3d4b4eb1fb8a745c2849840b1b4c3d89d
Reviewed-on: https://chromium-review.googlesource.com/c/1477695Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635571}