Add a WebContentsImpl API to snapshot the accessibility tree.

content/renderer/accessibility/renderer_accessibility.cc

Index: content/renderer/accessibility/renderer_accessibility.cc
diff --git a/content/renderer/accessibility/renderer_accessibility.cc b/content/renderer/accessibility/renderer_accessibility.cc
index b8152bac21e896b281d16e09ec84e0fc6c121140..12a59d3a1c38df477770dcf147a662876838f3df 100644
--- a/content/renderer/accessibility/renderer_accessibility.cc
+++ b/content/renderer/accessibility/renderer_accessibility.cc
@@ -25,11 +25,29 @@ using blink::WebLocalFrame;
 using blink::WebNode;
 using blink::WebPoint;
 using blink::WebRect;
+using blink::WebScopedAXContext;
 using blink::WebSettings;
 using blink::WebView;
 
 namespace content {
 
+// static
+void RendererAccessibility::SnapshotAccessibilityTree(
+    RenderFrameImpl* render_frame,
+    ui::AXTreeUpdate* response) {
+  DCHECK(render_frame);
+  DCHECK(response);
+  if (!render_frame->GetWebFrame())
+    return;
+
+  WebDocument document = render_frame->GetWebFrame()->document();
+  WebScopedAXContext context(document);
+  BlinkAXTreeSource tree_source(render_frame);
+  tree_source.SetRoot(context.root());
+  ui::AXTreeSerializer<blink::WebAXObject> serializer(&tree_source);
+  serializer.SerializeChanges(context.root(), response);
+}
+
 RendererAccessibility::RendererAccessibility(RenderFrameImpl* render_frame)
     : RenderFrameObserver(render_frame),
       render_frame_(render_frame),