Implement initial support for nesting one ax tree in another.

content/browser/accessibility/browser_accessibility.h

Index: content/browser/accessibility/browser_accessibility.h
diff --git a/content/browser/accessibility/browser_accessibility.h b/content/browser/accessibility/browser_accessibility.h
index 17f70caf675fe7aa1a983d4e057d90e7d1f26f6e..8ddbf48f2eaa26daa1d62978a84b97469f9fc1cd 100644
--- a/content/browser/accessibility/browser_accessibility.h
+++ b/content/browser/accessibility/browser_accessibility.h
@@ -245,6 +245,12 @@ class CONTENT_EXPORT BrowserAccessibility {
   // Append the text from this node and its children.
   std::string GetTextRecursive() const;
 
+  // Identifies the given frame id as the only child of this node, so
+  // any call to PlatformChildCount/PlatformGetChild will use
+  // FrameTreeAccessibility to retrieve the BrowserAccessibilityManager
+  // of the child frame and return its root node as this node's child.
+  void SetChildFrameId(uint32 child_frame_id);
+
  protected:
   BrowserAccessibility();
 
@@ -262,7 +268,8 @@ class CONTENT_EXPORT BrowserAccessibility {
   std::string name_;
   std::string value_;
 
- private:
+  uint32 child_frame_id_;
+
   DISALLOW_COPY_AND_ASSIGN(BrowserAccessibility);
 };