Fix race condition causing DCHECK(ack_pending_) to trip.

content/renderer/accessibility/render_accessibility_impl_browsertest.cc

 // Copyright (c) 2012 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 <tuple>
 
 #include "base/macros.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/time/time.h"
 #include "build/build_config.h"
@@ skipping 57 matching lines @@
 
   void SetMode(AccessibilityMode mode) {
     frame()->OnSetAccessibilityMode(mode);
   }
 
   void GetAllAccEvents(
       std::vector<AccessibilityHostMsg_EventParams>* param_list) {
     const IPC::Message* message =
         sink_->GetUniqueMessageMatching(AccessibilityHostMsg_Events::ID);
     ASSERT_TRUE(message);
-    std::tuple<std::vector<AccessibilityHostMsg_EventParams>, int> param;
+    std::tuple<std::vector<AccessibilityHostMsg_EventParams>, int, int> param;
     AccessibilityHostMsg_Events::Read(message, &param);
     *param_list = std::get<0>(param);
   }
 
   void GetLastAccEvent(
       AccessibilityHostMsg_EventParams* params) {
     std::vector<AccessibilityHostMsg_EventParams> param_list;
     GetAllAccEvents(&param_list);
     ASSERT_GE(param_list.size(), 1U);
     *params = param_list[0];
@@ skipping 244 matching lines @@
   GetLastAccEvent(&event);
   ASSERT_EQ(5U, event.update.nodes.size());
 
   EXPECT_EQ(body.axID(), event.update.nodes[0].id);
   EXPECT_EQ(text_1.axID(), event.update.nodes[1].id);
   // The third event is to update text_2, but its id changes
   // so we don't have a test expectation for it.
 }
 
 }  // namespace content