Fix race condition causing DCHECK(ack_pending_) to trip.

content/renderer/accessibility/render_accessibility_impl.h

Index: content/renderer/accessibility/render_accessibility_impl.h
diff --git a/content/renderer/accessibility/render_accessibility_impl.h b/content/renderer/accessibility/render_accessibility_impl.h
index bd7e0cb9b4884ddfae23cf78f6cf4df4dabb6155..363847a3d0e8287ab95efdfb7846b99ae9b0dd39 100644
--- a/content/renderer/accessibility/render_accessibility_impl.h
+++ b/content/renderer/accessibility/render_accessibility_impl.h
@@ -115,7 +115,7 @@ class CONTENT_EXPORT RenderAccessibilityImpl
 
   // Handlers for messages from the browser to the renderer.
   void OnDoDefaultAction(int acc_obj_id);
-  void OnEventsAck();
+  void OnEventsAck(int ack_token);
   void OnFatalError();
   void OnHitTest(gfx::Point point);
   void OnSetAccessibilityFocus(int acc_obj_id);
@@ -168,6 +168,8 @@ class CONTENT_EXPORT RenderAccessibilityImpl
   // We need to return this token in the next IPC.
   int reset_token_;
 
+  int ack_token_;
+
   // So we can queue up tasks to be executed later.
   base::WeakPtrFactory<RenderAccessibilityImpl> weak_factory_;