Make tree serialization more robust and add more unit tests.

ui/accessibility/ax_tree_source.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_SOURCE_H_
 #define UI_ACCESSIBILITY_AX_TREE_SOURCE_H_
 
 #include "ui/accessibility/ax_node_data.h"
 
 namespace ui {
 
 // An AXTreeSource is an abstract interface for a serializable
 // accessibility tree. The tree may be in some other format or
 // may be computed dynamically, but maintains the properties that
 // it's a strict tree, it has a unique id for each node, and all
 // of the accessibility information about a node can be serialized
 // as an AXNodeData. This is the primary interface to use when
 // an accessibility tree will be sent over an IPC before being
 // consumed.
 template<class AXNodeSource>
 class AX_EXPORT AXTreeSource {
  public:
   virtual ~AXTreeSource() {}
+  virtual AXNodeSource* GetRoot() const = 0;
   virtual AXNodeSource* GetFromId(int32 id) const = 0;
   virtual int32 GetId(const AXNodeSource* node) const = 0;
   virtual int GetChildCount(const AXNodeSource* node) const = 0;
   virtual AXNodeSource* GetChildAtIndex(const AXNodeSource* node, int index)
       const = 0;
-
-  // Returns the id of this node's parent, or 0 if it doesn't have a parent.
-  virtual int32 GetParentId(const AXNodeSource* node) const = 0;
+  virtual AXNodeSource* GetParent(const AXNodeSource* node) const = 0;
 
   // Serialize one node in the tree.
   virtual void SerializeNode(
       const AXNodeSource* node, AXNodeData* out_data) const = 0;
 
  protected:
   AXTreeSource() {}
 };
 
 }  // namespace ui
 
 #endif  // UI_ACCESSIBILITY_AX_TREE_SOURCE_H_