Add a WebContentsImpl API to snapshot the accessibility tree.

content/renderer/render_frame_impl.cc

Index: content/renderer/render_frame_impl.cc
diff --git a/content/renderer/render_frame_impl.cc b/content/renderer/render_frame_impl.cc
index 93029e3f14a69e9a308fdcc205568f9ac5cfa18c..bf54e6da56e6e131977dedb7de1ed271261160c2 100644
--- a/content/renderer/render_frame_impl.cc
+++ b/content/renderer/render_frame_impl.cc
@@ -1049,6 +1049,8 @@ bool RenderFrameImpl::OnMessageReceived(const IPC::Message& msg) {
                         OnAddStyleSheetByURL)
     IPC_MESSAGE_HANDLER(FrameMsg_SetAccessibilityMode,
                         OnSetAccessibilityMode)
+    IPC_MESSAGE_HANDLER(AccessibilityMsg_SnapshotTree,
+                        OnSnapshotAccessibilityTree)
     IPC_MESSAGE_HANDLER(FrameMsg_DisownOpener, OnDisownOpener)
     IPC_MESSAGE_HANDLER(FrameMsg_CommitNavigation, OnCommitNavigation)
     IPC_MESSAGE_HANDLER(FrameMsg_DidUpdateSandboxFlags, OnDidUpdateSandboxFlags)
@@ -1579,6 +1581,13 @@ void RenderFrameImpl::OnSetAccessibilityMode(AccessibilityMode new_mode) {
     renderer_accessibility_ = new RendererAccessibility(this);
 }
 
+void RenderFrameImpl::OnSnapshotAccessibilityTree(int callback_id) {
+  ui::AXTreeUpdate response;
+  RendererAccessibility::SnapshotAccessibilityTree(this, &response);
+  Send(new AccessibilityHostMsg_SnapshotResponse(
+      routing_id_, callback_id, response));
+}
+
 void RenderFrameImpl::OnDisownOpener() {
   // TODO(creis): We should only see this for main frames for now.  To support
   // disowning the opener on subframes, we will need to move WebContentsImpl's