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Hypertext is updated when a node changes, or on a parent of a
textnode that changes. There are some cases where the hypertexts
do not match the available children.

In some cases this results in returning a null hyperlink, because
cached data in AXHyperText was stale. A DumpWithoutCrashing() was
added to help narrow these situations down, and the situation occurs
quite often.

The problem is that when AXPlatformNodeBase::UpdateComputedHypertext()
is called, IsLeaf() is not updated nor are the platform children.
This occurs because although the SendPendingAccessibiityEvents()
has sent an updated root, the change for the root is not merged
with other changes (TreeUpdatesCanBeMerged() returns false for any
root update, even if it has the same id as the previous root).
This causes all updates to be processed separately. When the root is
processed, other updates have not yet occurred and the root object
still appears to be a leaf.

Other possibilities to fix this:
- Do not send root_id in an update unless it's actually a new root.
This may not catch all cases of the issue, but perhaps it's a good
thing to do anyway.
- Allow updates to be merged, even when there is a change on the root.
- Compute hypertext on demand and memoize, so that it's computed after
all atomic updates are finished
- Instead of calling OnAtomicUpdateFinished() at the end of
AXTree::Unserialize(), return the updates back to the caller and allow
the caller to collect the updates and process a single call to
OnAtomicUpdateFinished();
With any of these alternate approaches, it should be possible to
remove the logic to ensure hypertext updates on parents in
CollectChangedNodesAndParentsForAtomicUpdate(), because the
renderer already sends parents nodes to the serializer
with ChildrenChanged or TextChanged().

Bug: 1157731
Change-Id: I4e9eefa9b0f90d80daa1b7db2305ca92caaf32b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2587371Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836717}