Mitigation for Accessibility Trees initialized with root with id != 1

If the render-side tree was initialized with a root with an id != 1,
the first update won't match the tree created on the browser process
by CreateEmptyDocument. In this case, the update won't be able to
set the right node_id_to_clear to the correct root. This will cause
the "new" tree to be treated as an update, and a reparent.

To mitigate this, if node_id_to_clear was set and the update's
root_id doesn't match the old_root_id, we can assume that the
update meant to replace the root.

Bug: 1131848
Change-Id: Ia84c5a893a7b4846e0d81255455244171ec1d3d2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2598724
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Kurt Catti-Schmidt <kschmi@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#838960}

• ui/accessibility/ax_tree.cc[diff]
• ui/accessibility/ax_tree_unittest.cc[diff]