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In two recent changes, we added some calls to
AXTreeSerializer::DeleteClientSubtree. This is often a great way to
ensure that portions of the tree are re-serialized.

However, what can happen is that there can be reparenting within a subtree,
and the client doesn't clear out the subtree first before unserializing it,
leading to an unserialization error.

This change introduces the concept of invalidating a subtree
instead of deleting it. That ensures the correct nodes are
re-serialized, while also allowing the code that checks for
reparenting.

Includes a regression test.

Bug: 866293
Change-Id: I257f1a0360d73d2f42dff56fe84a52034941a63b
Reviewed-on: https://chromium-review.googlesource.com/1152180
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580703}