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When a node in the current group is removed we should check that
the current group is valid, and re-calculate its children.

This change also has some other bugfixes to improve stability
around edgecases.

AX-Relnotes: N/A
Bug: 1105196, 1106575
Change-Id: I0461150aadc42d5bd01099d96bf69d8d1f71119a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2303056Reviewed-by: Anastasia Helfinstein <anastasi@google.com>
Commit-Queue: Katie Dektar <katie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789643}