Add a WebContentsImpl API to snapshot the accessibility tree.

content/renderer/accessibility/blink_ax_tree_source.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CONTENT_RENDERER_ACCESSIBILITY_BLINK_AX_TREE_SOURCE_H_
 #define CONTENT_RENDERER_ACCESSIBILITY_BLINK_AX_TREE_SOURCE_H_
 
 #include "third_party/WebKit/public/web/WebAXObject.h"
 #include "ui/accessibility/ax_node_data.h"
 #include "ui/accessibility/ax_tree_source.h"
 
 namespace content {
 
 class RenderFrameImpl;
 
 class BlinkAXTreeSource
     : public ui::AXTreeSource<blink::WebAXObject> {
  public:
   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.
   void CollectChildFrameIdMapping(
       std::map<int32, int>* node_to_frame_routing_id_map,
       std::map<int32, int>* node_to_browser_plugin_instance_id_map);
 
   // Walks up the ancestor chain to see if this is a descendant of the root.
   bool IsInTree(blink::WebAXObject node) const;
 
@@ skipping 15 matching lines @@
                      ui::AXNodeData* out_data) const override;
   bool IsValid(blink::WebAXObject node) const override;
   bool IsEqual(blink::WebAXObject node1,
                blink::WebAXObject node2) const override;
   blink::WebAXObject GetNull() const override;
 
   blink::WebDocument GetMainDocument() const;
 
  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_;
 };
 
 }  // namespace content
 
 #endif  // CONTENT_RENDERER_ACCESSIBILITY_BLINK_AX_TREE_SOURCE_H_