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When navigating the unignored nodes in the accessibility tree and when retrieving
any node information that needs to be computed by traversing the accessibility tree,
we want to ensure that a tree update is not in progress.
This patch adds such DCHECKs in more methods in AXNode.
The only methods that are not guarded are those that get or set data members directly,
e.g. IndexInParent() and SetIndexInParent(), and those that are used to traverse the
whole tree, such as parent() and NextSibling(), as those may be called from AXTree while an update is in progress, in order to
populate the AXNode.

R=dmazzoni@chromium.org, aleventhal@chromium.org

AX-Relnotes: n/a.
Change-Id: I92c34991bf4b0583dd09bb061db9ff35c8014e9d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2623288
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842714}