Add AXTreeDelegate and refactor other AXTree classes slightly.

ui/accessibility/ax_tree_source.h

Index: ui/accessibility/ax_tree_source.h
diff --git a/ui/accessibility/ax_tree_source.h b/ui/accessibility/ax_tree_source.h
index 35bdbb711c811f7cb4385778280e9ba12a296032..0c99c838a5f1395909f6dfdff117d7dab87d49d8 100644
--- a/ui/accessibility/ax_tree_source.h
+++ b/ui/accessibility/ax_tree_source.h
@@ -5,6 +5,8 @@
 #ifndef UI_ACCESSIBILITY_AX_TREE_SOURCE_H_
 #define UI_ACCESSIBILITY_AX_TREE_SOURCE_H_
 
+#include <vector>
+
 #include "ui/accessibility/ax_node_data.h"
 
 namespace ui {
@@ -17,21 +19,41 @@ namespace ui {
 // 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>
+template<typename 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;
-  virtual AXNodeSource* GetParent(const AXNodeSource* node) const = 0;
+
+  // Get the root of the tree.
+  virtual AXNodeSource GetRoot() const = 0;
+
+  // Get a node by its id. If no node by that id exists in the tree, return a
+  // null node, i.e. one that will return false if you call IsValid on it.
+  virtual AXNodeSource GetFromId(int32 id) const = 0;
+
+  // Return the id of a node. All ids must be positive integers.
+  virtual int32 GetId(AXNodeSource node) const = 0;
+
+  // Append all children of |node| to |out_children|.
+  virtual void GetChildren(AXNodeSource node,
+                           std::vector<AXNodeSource>* out_children) const = 0;
+
+  // Get the parent of |node|.
+  virtual AXNodeSource GetParent(AXNodeSource node) const = 0;
+
+  // Returns true if |node| is valid, and false if it's a null pointer or a
+  // node object representing the null pointer.
+  virtual bool IsValid(AXNodeSource node) const = 0;
+
+  // Returns true if two nodes are equal.
+  virtual bool IsEqual(AXNodeSource node1,
+                       AXNodeSource node2) const = 0;
+
+  // Return a AXNodeSource representing null.
+  virtual AXNodeSource GetNull() const = 0;
 
   // Serialize one node in the tree.
-  virtual void SerializeNode(
-      const AXNodeSource* node, AXNodeData* out_data) const = 0;
+  virtual void SerializeNode(AXNodeSource node, AXNodeData* out_data) const = 0;
 
  protected:
   AXTreeSource() {}