GetUnignoredSelection causes DCHECK in GetManagerForChildTree

GetUnignoredSelection (introduced in crrev.com/c/1691559) introduced a
new selection API built on AXPosition. Exposing AXPosition in such
a broad way revealed limitations of AXPosition on specific
platforms. In particular, ChromeOS doesn't have a
BrowserAccessibilityManager, so workarounds were necessary for selection
to work there.

This DCHECK is another of those exceptions - BrowserAccessibilityManager
isn't available on ChromeOS, so the DCHECK is too aggressive to be a
universal expectation. The callers of the function already handle it
returning nullptr, so it's safe to simply return nullptr if there's
no BrowserAccessibilityManager available instead of DCHECKing.

Bug: 995807
Change-Id: I43d21337ce071045d0653be39e10f5003402675e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1773209
Commit-Queue: Kurt Catti-Schmidt <kschmi@microsoft.com>
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#690956}

ui/accessibility/ax_tree_manager_map.cc

@@ -48,9 +48,13 @@ AXTreeManager* AXTreeManagerMap::GetManagerForChildTree(
   AXTreeManager* child_tree_manager =
       AXTreeManagerMap::GetInstance().GetManager(child_tree_id);
 
-  DCHECK(child_tree_manager &&
-         child_tree_manager->GetParentNodeFromParentTreeAsAXNode()->id() ==
-             parent_node.id());
+  // Some platforms do not use AXTreeManagers, so child trees don't exist in
+  // the browser process.
+  if (!child_tree_manager)
+    return nullptr;
+
+  DCHECK(child_tree_manager->GetParentNodeFromParentTreeAsAXNode()->id() ==
+         parent_node.id());
 
   return child_tree_manager;
 }