Add a WebContentsImpl API to snapshot the accessibility tree.

content/browser/web_contents/web_contents_impl.h

Index: content/browser/web_contents/web_contents_impl.h
diff --git a/content/browser/web_contents/web_contents_impl.h b/content/browser/web_contents/web_contents_impl.h
index 3c45bd55546ddbde5a4a203c9cf76de45c85b332..3d4656acb2665d4f83bcefbc5d843f9f9ce31aaa 100644
--- a/content/browser/web_contents/web_contents_impl.h
+++ b/content/browser/web_contents/web_contents_impl.h
@@ -204,6 +204,12 @@ class CONTENT_EXPORT WebContentsImpl
   // have been removed.
   void RemoveAccessibilityMode(AccessibilityMode mode);
 
+  // Request a one-time snapshot of the accessibility tree without changing
+  // the accessibility mode.
+  typedef base::Callback<void(const ui::AXTreeUpdate&)>
+      AXTreeSnapshotCallback;
+  void RequestAXTreeSnapshot(AXTreeSnapshotCallback callback);
+
   // Clear the navigation transition data when the user navigates back to Chrome
   // from a native app.
   void ClearNavigationTransitionData();