DevTools: report adding mutation event listener as a violations.

third_party/WebKit/Source/core/events/EventTarget.cpp

Index: third_party/WebKit/Source/core/events/EventTarget.cpp
diff --git a/third_party/WebKit/Source/core/events/EventTarget.cpp b/third_party/WebKit/Source/core/events/EventTarget.cpp
index c3d395b70091abac31014faee6c00a895e1481b3..ebf6e7d513d2acd7e3a3a6b2d01caff95ae10caa 100644
--- a/third_party/WebKit/Source/core/events/EventTarget.cpp
+++ b/third_party/WebKit/Source/core/events/EventTarget.cpp
@@ -336,6 +336,16 @@ void EventTarget::AddedEventListener(
       UseCounter::Count(executing_window->document(),
                         UseCounter::kSlotChangeEventAddListener);
     }
+  } else if (EventUtil::IsDOMMutationEventType(event_type)) {
+    if (ExecutionContext* context = GetExecutionContext()) {
+      String message_text = String::Format(
+          "Added synchronous DOM mutation listener to a '%s' event. "
+          "Consider using MutationObserver to make the page more responsive.",
+          event_type.GetString().Utf8().data());
+      PerformanceMonitor::ReportGenericViolation(
+          context, PerformanceMonitor::kDiscouragedAPIUse, message_text, 0,
+          nullptr);
+    }
   }
 }