Re-land: Reimplement automation API on top of C++-backed AXTree.

ui/accessibility/ax_tree.cc

Index: ui/accessibility/ax_tree.cc
diff --git a/ui/accessibility/ax_tree.cc b/ui/accessibility/ax_tree.cc
index d07f239b79d9a292b1221239067b71ee2b715e03..60fa8880ad7c72f01a792f894c8c5d1e608ebea2 100644
--- a/ui/accessibility/ax_tree.cc
+++ b/ui/accessibility/ax_tree.cc
@@ -204,9 +204,12 @@ bool AXTree::UpdateNode(const AXNodeData& src,
   // Update the root of the tree if needed.
   if ((src.role == AX_ROLE_ROOT_WEB_AREA || src.role == AX_ROLE_DESKTOP) &&
       (!root_ || root_->id() != src.id)) {
-    if (root_)
-      DestroySubtree(root_, update_state);
+    // Make sure root_ always points to something valid, even inside
+    // DestroySubtree.
+    AXNode* old_root = root_;
     root_ = node;
+    if (old_root)
+      DestroySubtree(old_root, update_state);
   }
 
   return success;
@@ -221,11 +224,11 @@ void AXTree::DestroySubtree(AXNode* node,
 
 void AXTree::DestroyNodeAndSubtree(AXNode* node,
                                    AXTreeUpdateState* update_state) {
+  if (delegate_)
+    delegate_->OnNodeWillBeDeleted(this, node);
   id_map_.erase(node->id());
   for (int i = 0; i < node->child_count(); ++i)
     DestroyNodeAndSubtree(node->ChildAtIndex(i), update_state);
-  if (delegate_)
-    delegate_->OnNodeWillBeDeleted(this, node);
   if (update_state) {
     update_state->pending_nodes.erase(node);
   }