[Reading mode] Handle invalid selection object.

Sometimes, AXTree#GetUnignoredSelection can return a selection object
with ids that are technically valid but aren't actually in the AXTree.

This can cause crashes which reading mode is failing gracefully with
in stable builds, but this CL adds a check to has_selection() to
verify that the selection object is actually in the tree. This
is a more robust fix because instead of just failing gracefully,
reading mode recognizes that selection is not actually valid.

The root cause of this discrepancy seems related to iframes and
child tree ids.

Fixed: 418455351
Change-Id: I37b34739b2a63780a4623df62d91b82d700e9d48
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6579790
Commit-Queue: Lauren Winston <lwinston@google.com>
Reviewed-by: Kristi Saney <kristislee@google.com>
Cr-Commit-Position: refs/heads/main@{#1464936}