ax_ui: crash on getting unignored selection

It's either MSAA caller does not have accessibility fully enabled, the case when a tree is not registered with a tree manager, or when a tree stays longer than it's registered with a tree manager. Assert in the former case to detect if this is a case. Return the selection as is in the latter case: the tree goes away and we cannot guarantee any data validity for it.

Regressed by crrev.com/c/3863567

(cherry picked from commit 68da7b4)

Change-Id: I83ca7b5144c81b78b16cdc7748d015a2a12fe984
Bug: 1371027
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3934862
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Cr-Original-Commit-Position: refs/heads/main@{#1055870}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3937765
Reviewed-by: Krishna Govind <govind@chromium.org>
Commit-Queue: Prudhvikumar Bommana <pbommana@google.com>
Owners-Override: Prudhvikumar Bommana <pbommana@google.com>
Cr-Commit-Position: refs/branch-heads/5343@{#3}
Cr-Branched-From: d1a9027-refs/heads/main@{#1055528}

ui/accessibility/ax_selection.cc

@@ -97,8 +97,11 @@ AXSelection::AXSelection(const AXTree& tree)
       tree_id_(tree.GetAXTreeID()) {}
 
 AXSelection& AXSelection::ToUnignoredSelection() {
+  DCHECK_NE(tree_id_, AXTreeIDUnknown())
+      << "Tree is not registered with a tree manager";
   const AXTreeManager* manager = AXTreeManager::FromID(tree_id_);
-  DCHECK(manager);
+  if (!manager)
+    return *this;
 
   // If one of the selection endpoints is invalid, then the other endpoint
   // should also be unset.