Fix race condition causing DCHECK(ack_pending_) to trip.

content/browser/frame_host/render_frame_host_impl.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CONTENT_BROWSER_FRAME_HOST_RENDER_FRAME_HOST_IMPL_H_
 #define CONTENT_BROWSER_FRAME_HOST_RENDER_FRAME_HOST_IMPL_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 685 matching lines @@
   void OnDidChangeFrameOwnerProperties(int32_t frame_routing_id,
                                        const FrameOwnerProperties& properties);
   void OnUpdateTitle(const base::string16& title,
                      blink::WebTextDirection title_direction);
   void OnUpdateEncoding(const std::string& encoding);
   void OnBeginNavigation(const CommonNavigationParams& common_params,
                          const BeginNavigationParams& begin_params);
   void OnDispatchLoad();
   void OnAccessibilityEvents(
       const std::vector<AccessibilityHostMsg_EventParams>& params,
-      int reset_token);
+      int reset_token,
+      int ack_token);
   void OnAccessibilityLocationChanges(
       const std::vector<AccessibilityHostMsg_LocationChangeParams>& params);
   void OnAccessibilityFindInPageResult(
       const AccessibilityHostMsg_FindInPageResultParams& params);
   void OnAccessibilityChildFrameHitTestResult(const gfx::Point& point,
                                               int hit_obj_id);
   void OnAccessibilitySnapshotResponse(
       int callback_id,
       const AXContentTreeUpdate& snapshot);
   void OnToggleFullscreen(bool enter_fullscreen);
@@ skipping 340 matching lines @@
 
   // NOTE: This must be the last member.
   base::WeakPtrFactory<RenderFrameHostImpl> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(RenderFrameHostImpl);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_FRAME_HOST_RENDER_FRAME_HOST_IMPL_H_