bug 1162543 - IDs not mapping to accessibles shouldn't cause ipc call…

…s to fail r=davidb

This is expected to happen when a accessible in the child process is shutdown,
     but the parent calls a method with its ID before learning it has been shut
     down.

accessible/ipc/DocAccessibleChild.cpp

@@ -224,7 +224,7 @@ DocAccessibleChild::RecvRelationByType(const uint64_t& aID,
 {
   Accessible* acc = mDoc->GetAccessibleByUniqueID((void*)aID);
   if (!acc)
-    return false;
+    return true;
 
   auto type = static_cast<RelationType>(aType);
   Relation rel = acc->RelationByType(type);
@@ -256,8 +256,8 @@ DocAccessibleChild::RecvRelations(const uint64_t& aID,
                                   nsTArray<RelationTargets>* aRelations)
 {
   Accessible* acc = mDoc->GetAccessibleByUniqueID((void*)aID);
-  if (!aID)
-    return false;
+  if (!acc)
+    return true;
 
 #define RELATIONTYPE(gecko, s, a, m, i) AddRelation(acc, RelationType::gecko, aRelations);