Bug 1996506 - Don't climb above document in GetNonGenericParent. r=Jamie

Also don't allow climbing into local accessibles off main thread, and
assert if we do.

Differential Revision: https://phabricator.services.mozilla.com/D272729

Author: eeejay

accessible/basetypes/Accessible.h

@@ -705,11 +705,12 @@ class Accessible {
   }
 
   /**
-   * Returns the nearest ancestor which is not a generic element.
+   * Returns the nearest ancestor in the document which is not a generic
+   * element.
    */
   Accessible* GetNonGenericParent() const {
     for (Accessible* parent = Parent(); parent; parent = parent->Parent()) {
-      if (!parent->IsGeneric()) {
+      if (parent->IsDoc() || !parent->IsGeneric()) {
         return parent;
       }
     }