Add a WebContentsImpl API to snapshot the accessibility tree.

content/renderer/accessibility/blink_ax_tree_source.h

Index: content/renderer/accessibility/blink_ax_tree_source.h
diff --git a/content/renderer/accessibility/blink_ax_tree_source.h b/content/renderer/accessibility/blink_ax_tree_source.h
index b08108b12e576a2446774cba67f37638906f058b..38963fd44d1682669fa4b5ea0f889d435a0e6409 100644
--- a/content/renderer/accessibility/blink_ax_tree_source.h
+++ b/content/renderer/accessibility/blink_ax_tree_source.h
@@ -19,6 +19,8 @@ class BlinkAXTreeSource
   BlinkAXTreeSource(RenderFrameImpl* render_frame);
   ~BlinkAXTreeSource() override;
 
+  void SetRoot(blink::WebAXObject root);
+
   // Call this to have BlinkAXTreeSource collect a mapping from
   // node ids to the frame routing id for an out-of-process iframe during
   // calls to SerializeNode.
@@ -54,6 +56,7 @@ class BlinkAXTreeSource
 
  private:
   RenderFrameImpl* render_frame_;
+  blink::WebAXObject root_;
   std::map<int32, int>* node_to_frame_routing_id_map_;
   std::map<int32, int>* node_to_browser_plugin_instance_id_map_;
   int accessibility_focus_id_;