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In http://crrev.com/c/1152180, we introduced
AXTreeSerializer::InvalidateSubtree. This replaced
AXTreeSerializer::DeleteClientSubtree, which was problematic and led
to subtle, hard-to-catch unserialization errors.

This patch adds comprehensive tests for InvalidateSubtree by introducing it
into the "generated tree" tests, which try every permutation of trees with
up to a certain number of nodes and every possible change to those trees.
Now those tests also try every possible way to invalidate a subtree before
serializing.

This uncovered one subtle bug: the code to detect reparenting also needed
to check the 'invalid' flag and recursively check invalid children.

Bug: 870661, 866293
Change-Id: I45313b712e50db4fba16a5d55f72ecd440f04335
Reviewed-on: https://chromium-review.googlesource.com/1180091Reviewed-by: David Tseng <dtseng@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584643}