Fix race condition causing DCHECK(ack_pending_) to trip.

content/browser/frame_host/render_frame_host_impl.cc

 // 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.
 
 #include "content/browser/frame_host/render_frame_host_impl.h"
 
 #include <utility>
 
 #include "base/bind.h"
 #include "base/command_line.h"
@@ skipping 1772 matching lines @@
       frame_tree_node_->IsMainFrame()
           ? render_view_host_->GetWidget()->GetView()
           : frame_tree_node_->frame_tree()
                 ->GetMainFrame()
                 ->render_view_host_->GetWidget()
                 ->GetView());
 }
 
 void RenderFrameHostImpl::OnAccessibilityEvents(
     const std::vector<AccessibilityHostMsg_EventParams>& params,
-    int reset_token) {
+    int reset_token, int ack_token) {
   // Don't process this IPC if either we're waiting on a reset and this
   // IPC doesn't have the matching token ID, or if we're not waiting on a
   // reset but this message includes a reset token.
   if (accessibility_reset_token_ != reset_token) {
-    Send(new AccessibilityMsg_Events_ACK(routing_id_));
+    Send(new AccessibilityMsg_Events_ACK(routing_id_, ack_token));
     return;
   }
   accessibility_reset_token_ = 0;
 
   RenderWidgetHostViewBase* view = GetViewForAccessibility();
 
   AccessibilityMode accessibility_mode = delegate_->GetAccessibilityMode();
   if ((accessibility_mode != AccessibilityModeOff) && view && is_active()) {
     if (accessibility_mode & AccessibilityModeFlagPlatform)
       GetOrCreateBrowserAccessibilityManager();
@@ skipping 49 matching lines @@
         } else {
           CHECK(ax_tree_for_testing_->Unserialize(detail.update))
               << ax_tree_for_testing_->error();
         }
         accessibility_testing_callback_.Run(this, detail.event_type, detail.id);
       }
     }
   }
 
   // Always send an ACK or the renderer can be in a bad state.
-  Send(new AccessibilityMsg_Events_ACK(routing_id_));
+  Send(new AccessibilityMsg_Events_ACK(routing_id_, ack_token));
 }
 
 void RenderFrameHostImpl::OnAccessibilityLocationChanges(
     const std::vector<AccessibilityHostMsg_LocationChangeParams>& params) {
   if (accessibility_reset_token_)
     return;
 
   RenderWidgetHostViewBase* view = static_cast<RenderWidgetHostViewBase*>(
       render_view_host_->GetWidget()->GetView());
   if (view && is_active()) {
@@ skipping 1177 matching lines @@
   web_bluetooth_service_->SetClientConnectionErrorHandler(base::Bind(
       &RenderFrameHostImpl::DeleteWebBluetoothService, base::Unretained(this)));
   return web_bluetooth_service_.get();
 }
 
 void RenderFrameHostImpl::DeleteWebBluetoothService() {
   web_bluetooth_service_.reset();
 }
 
 }  // namespace content