Reland "Refined DCHECK preventing multiple event forwarders in VA tree path"

ui/android/view_android_unittests.cc

Index: ui/android/view_android_unittests.cc
diff --git a/ui/android/view_android_unittests.cc b/ui/android/view_android_unittests.cc
index 77c9bcae16534428660684bb80c2c99af788a641..78a01f68cd03b044181737eb4f5665e9552c05c0 100644
--- a/ui/android/view_android_unittests.cc
+++ b/ui/android/view_android_unittests.cc
@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+#include "base/test/gtest_util.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "ui/android/event_forwarder.h"
 #include "ui/android/view_android.h"
@@ -166,4 +167,37 @@ TEST_F(ViewAndroidBoundsTest, MatchesViewsWithOffset) {
   ExpectHit(client3_);
 }
 
+TEST(ViewAndroidTest, ChecksMultipleEventForwarders) {
+  ViewAndroid parent;
+  ViewAndroid child;
+  parent.GetEventForwarder();
+  child.GetEventForwarder();
+  EXPECT_DCHECK_DEATH(parent.AddChild(&child));
+
+  ViewAndroid parent2;
+  ViewAndroid child2;
+  parent2.GetEventForwarder();
+  parent2.AddChild(&child2);
+  EXPECT_DCHECK_DEATH(child2.GetEventForwarder());
+
+  ViewAndroid window;
+  ViewAndroid wcv1, wcv2;
+  ViewAndroid rwhv1a, rwhv1b, rwhv2;
+  wcv1.GetEventForwarder();
+  wcv2.GetEventForwarder();
+
+  window.AddChild(&wcv1);
+  wcv1.AddChild(&rwhv1a);
+  wcv1.AddChild(&rwhv1b);
+
+  wcv2.AddChild(&rwhv2);
+
+  // window should be able to add wcv2 since there's only one event forwarder
+  // in the path window - wcv2* - rwvh2
+  window.AddChild(&wcv2);
+
+  // Additional event forwarder will cause failure.
+  EXPECT_DCHECK_DEATH(rwhv2.GetEventForwarder());
+}
+
 }  // namespace ui