DevTools: report adding mutation event listener as a violations.

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

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  *           (C) 2007, 2008 Nikolas Zimmermann <zimmermann@kde.org>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ skipping 318 matching lines @@
   } else if (EventUtil::IsPointerEventType(event_type)) {
     if (LocalDOMWindow* executing_window = this->ExecutingWindow()) {
       UseCounter::Count(executing_window->document(),
                         UseCounter::kPointerEventAddListenerCount);
     }
   } else if (event_type == EventTypeNames::slotchange) {
     if (LocalDOMWindow* executing_window = this->ExecutingWindow()) {
       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);
+    }
   }
 }
 
 bool EventTarget::removeEventListener(const AtomicString& event_type,
                                       const EventListener* listener,
                                       bool use_capture) {
   EventListenerOptions options;
   options.setCapture(use_capture);
   return RemoveEventListenerInternal(event_type, listener, options);
 }
@@ skipping 426 matching lines @@
   // they have one less listener to invoke.
   if (d->firing_event_iterators) {
     for (const auto& iterator : *d->firing_event_iterators) {
       iterator.iterator = 0;
       iterator.end = 0;
     }
   }
 }
 
 }  // namespace blink