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Some web trees, in particular:
rootWebArea
  iframe
    ***rootWebArea
      iframe
        rootWebArea

*** will set its
tree->data().focus_id
but not either its
focus->GetStringAttribute(            ax::mojom::StringAttribute::kChildTreeId)
nor its
tree->data().focused_tree_id

The code assumed this more strict interpretation of the data, which appears to not work in this case. The topmost tree directly points to the innermost frame, skipping the frame in-between.

Fixed: 1038739
Test: manually on the snippet in the bug.
Change-Id: I5174cfcfabeb3bb4606d8636ab0f867d1310e52e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2080783Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746120}