Refined DCHECK preventing multiple event forwarders in VA tree path

ui/android/view_android_unittests.cc

 // Copyright 2017 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 "testing/gtest/include/gtest/gtest.h"
 #include "ui/android/event_forwarder.h"
 #include "ui/android/view_android.h"
 #include "ui/android/view_client.h"
+#include "ui/android/window_android.h"
 #include "ui/events/android/motion_event_android.h"
 
 namespace ui {
 
 using base::android::JavaParamRef;
 
 class TestViewClient : public ViewClient {
  public:
   TestViewClient() : handle_event_(true), called_(false) {}
 
@@ skipping 140 matching lines @@
   client1_.SetHandleEvent(false);
   GenerateTouchEventAt(40, 60);
   EXPECT_TRUE(client1_.EventCalled());
   ExpectHit(client2_);
 
   GenerateTouchEventAt(100, 70);
   EXPECT_TRUE(client1_.EventCalled());
   ExpectHit(client3_);
 }
 
+TEST(ViewAndroidTest, AddChildChecksMultipleEventForwarders) {
+  ViewAndroid view1;
+  ViewAndroid view2;
+  view1.GetEventForwarder();
+  view2.GetEventForwarder();
+  EXPECT_DEATH(view1.AddChild(&view2), "Check failed:");
+
+  WindowAndroid* window = WindowAndroid::CreateForTesting();
+  view2.RemoveFromParent();
+  window->AddChild(&view1);
+  window->AddChild(&view2);
+  window->Destroy(nullptr, nullptr);
+}
+
 }  // namespace ui