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..95ce3c25994165995e24cadee52c11fbee2d171b 100644
--- a/ui/android/view_android_unittests.cc
+++ b/ui/android/view_android_unittests.cc
@@ -166,4 +166,30 @@ TEST_F(ViewAndroidBoundsTest, MatchesViewsWithOffset) {
   ExpectHit(client3_);
 }
 
+TEST(ViewAndroidTest, ChecksMultipleEventForwarders) {
+  ViewAndroid parent;
+  ViewAndroid child;
+  parent.GetEventForwarder();
+  child.GetEventForwarder();
+  EXPECT_DEATH(parent.AddChild(&child), "Check failed:");
+
+  ViewAndroid window;
+  ViewAndroid wcv1, wcv2;
+  ViewAndroid rwhv1a, rwhv1b, rwhv2;
+  wcv1.GetEventForwarder();
+  wcv1.GetEventForwarder();
+
+  window.AddChild(&wcv1);
+  wcv1.AddChild(&rwhv1a);
+  wcv1.AddChild(&rwhv1b);
+  wcv2.AddChild(&rwhv2);
+
+  // window can add wcv2 since there's only one event forwarder at wcv2
+  // in the path window - wcv2 - rwvh2
+  window.AddChild(&wcv2);
+
+  // Both wcv1 & rwhv1 have event forwarders in the path window - wcv1 - rwhv1a.
+  EXPECT_DEATH(rwhv1a.GetEventForwarder(), "Check failed:");
+}
+
 }  // namespace ui