Make tree serialization more robust and add more unit tests.

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 383e70e3f0e25c15cbaab3ed132b50ff0f6a8b67..35bdbb711c811f7cb4385778280e9ba12a296032 100644
--- a/ui/accessibility/ax_tree_source.h
+++ b/ui/accessibility/ax_tree_source.h
@@ -21,14 +21,13 @@ 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(