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 389b8038d6392d5127623498dd1ced2638db87c0..c787692d431da03ca7ceac82e35b77ce59766774 100644
--- a/content/renderer/render_frame_impl.cc
+++ b/content/renderer/render_frame_impl.cc
@@ -1042,6 +1042,8 @@ 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)
@@ -1563,6 +1565,12 @@ void RenderFrameImpl::OnSetAccessibilityMode(AccessibilityMode new_mode) {
     renderer_accessibility_ = new RendererAccessibility(this);
 }
 
+void RenderFrameImpl::OnSnapshotAccessibilityTree(int token) {
+  ui::AXTreeUpdate response;
+  RendererAccessibility::SnapshotAccessibilityTree(this, &response);
+  Send(new AccessibilityHostMsg_SnapshotResponse(routing_id_, token, 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