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..be51ea98f8385148f987aedd353f77e5801a9c93 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 +19,24 @@ 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;
+  virtual AXNodeSource GetRoot() const = 0;
+  virtual AXNodeSource GetFromId(int32 id) const = 0;
+  virtual int32 GetId(AXNodeSource node) const = 0;
+  virtual void GetChildren(const AXNodeSource node,
+                           std::vector<AXNodeSource>* out_children) const = 0;
+  virtual AXNodeSource GetParent(const AXNodeSource node) const = 0;
+  virtual bool IsValid(const AXNodeSource node) const = 0;
+  virtual bool IsEqual(const AXNodeSource node1,
+                       const AXNodeSource node2) const = 0;
+  virtual AXNodeSource GetNull() const = 0;
 
   // Serialize one node in the tree.
   virtual void SerializeNode(
-      const AXNodeSource* node, AXNodeData* out_data) const = 0;
+      const AXNodeSource node, AXNodeData* out_data) const = 0;
 
  protected:
   AXTreeSource() {}