Make tree serialization more robust and add more unit tests.

ui/accessibility/ax_tree_update.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_ACCESSIBILITY_AX_TREE_UPDATE_H_
 #define UI_ACCESSIBILITY_AX_TREE_UPDATE_H_
 
 #include <vector>
 
 #include "ui/accessibility/ax_node_data.h"
 
 namespace ui {
 
 // An AXTreeUpdate is a serialized representation of an atomic change
 // to an AXTree. The sender and receiver must be in sync; the update
 // is only meant to bring the tree from a specific previous state into
 // its next state. Trying to apply it to the wrong tree should immediately
-// die with a fatal assertion. An AXTreeUpdate is just an ordered vector
-// of AXNodeData structures to be applied to the tree in order.
+// die with a fatal assertion.
+//
+// An AXTreeUpdate consists of an optional node id to clear (meaning
+// that all of that node's children and their descendants are deleted),
+// followed by an ordered vector of AXNodeData structures to be applied
+// to the tree in order.
 //
 // Suppose that the next AXNodeData to be applied is |node|. The following
 // invariants must hold:
-// 1. Either |node.id| is already in the tree, or else |node| is the new
-//        root of the tree and |node.role| == WebAXRoleRootWebArea.
+// 1. Either |node.id| is already in the tree, or else the tree is empty,
+//        |node| is the new root of the tree, and
+//        |node.role| == WebAXRoleRootWebArea.
 // 2. Every child id in |node.child_ids| must either be already a child
 //        of this node, or a new id not previously in the tree. It is not
 //        allowed to "reparent" a child to this node without first removing
 //        that child from its previous parent.
 // 3. When a new id appears in |node.child_ids|, the tree should create a
 //        new uninitialized placeholder node for it immediately. That
 //        placeholder must be updated within the same AXTreeUpdate, otherwise
 //        it's a fatal error. This guarantees the tree is always complete
 //        before or after an AXTreeUpdate.
 struct AX_EXPORT AXTreeUpdate {
   AXTreeUpdate();
   ~AXTreeUpdate();
 
+  // The id of a node to clear, before applying any updates,
+  // or 0 if no nodes should be cleared. Clearing a node means deleting
+  // all of its children and their descendants, but leaving that node in
+  // the tree. It's an error to clear a node but not subsequently update it
+  // as part of the tree update.
+  int node_id_to_clear;
+
+  // A vector of nodes to update, according to the rules above.
   std::vector<AXNodeData> nodes;
 
   // TODO(dmazzoni): location changes
 };
 
 }  // namespace ui
 
 #endif  // UI_ACCESSIBILITY_AX_TREE_UPDATE_H_