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 4920532a3f94bea97093bf2224f1d948465afddb..3a9fa71eb5741c597c212f0b61f4d8d2e00adb76 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);
@@ -171,6 +171,9 @@ class CONTENT_EXPORT RenderAccessibilityImpl
   // Whether we are processing a client-initiated action.
   bool during_action_;
 
+  // Token to send with event messages so we know when they're acknowledged.
+  int ack_token_;
+
   // So we can queue up tasks to be executed later.
   base::WeakPtrFactory<RenderAccessibilityImpl> weak_factory_;