Fix race condition causing DCHECK(ack_pending_) to trip.

content/renderer/accessibility/render_accessibility_impl_browsertest.cc

Index: content/renderer/accessibility/render_accessibility_impl_browsertest.cc
diff --git a/content/renderer/accessibility/render_accessibility_impl_browsertest.cc b/content/renderer/accessibility/render_accessibility_impl_browsertest.cc
index 77c180742c6f1aad7ba758d330ef98afea9c04c7..ebf2f516e23d3911b93d975b74f5d67dcb68582f 100644
--- a/content/renderer/accessibility/render_accessibility_impl_browsertest.cc
+++ b/content/renderer/accessibility/render_accessibility_impl_browsertest.cc
@@ -75,7 +75,7 @@ class RenderAccessibilityImplTest : public RenderViewTest {
     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);
   }