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This CL detects when the children's order has changed
in a tree update in order to invalidate the unignored
index in parent cache. Before this change, the added
test case would fail and could cause an infinite
loop in the tree walker in the extension.

Bug: 1005799
Test: Added unit test and manual testing of display assistant
Change-Id: I902e74b0b5e25b949486bc9d7edc208cdd2aa846
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1813521Reviewed-by: David Tseng <dtseng@chromium.org>
Commit-Queue: Randy Rossi <rmrossi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#698515}