Only show a beforeunload dialog if a frame has had a user gesture since its load.

third_party/WebKit/Source/core/dom/Document.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2011, 2012 Apple Inc. All
  * rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved.
  * (http://www.torchmobile.com/)
  * Copyright (C) 2008, 2009, 2011, 2012 Google Inc. All rights reserved.
@@ skipping 3153 matching lines @@
   BeforeUnloadEvent* before_unload_event = BeforeUnloadEvent::Create();
   before_unload_event->initEvent(EventTypeNames::beforeunload, false, true);
   load_event_progress_ = kBeforeUnloadEventInProgress;
   dom_window_->DispatchEvent(before_unload_event, this);
   load_event_progress_ = kBeforeUnloadEventCompleted;
   if (!before_unload_event->defaultPrevented())
     DefaultEventHandler(before_unload_event);
   if (!GetFrame() || before_unload_event->returnValue().IsNull())
     return true;
 
+  if (!GetFrame()->HasReceivedUserGesture()) {
+    AddConsoleMessage(ConsoleMessage::Create(
+        kJSMessageSource, kErrorMessageLevel,
+        "Blocked attempt to show a 'beforeunload' confirmation panel for a "
+        "frame that never had a user gesture since its load. "
+        "https://www.chromestatus.com/feature/5082396709879808"));
+    return true;
+  }
+
   if (did_allow_navigation) {
     AddConsoleMessage(ConsoleMessage::Create(
         kJSMessageSource, kErrorMessageLevel,
         "Blocked attempt to show multiple 'beforeunload' confirmation panels "
         "for a single navigation."));
     return true;
   }
 
   String text = before_unload_event->returnValue();
   if (chrome_client.OpenBeforeUnloadConfirmPanel(text, frame_, is_reload)) {
@@ skipping 3595 matching lines @@
 }
 
 void showLiveDocumentInstances() {
   WeakDocumentSet& set = liveDocumentSet();
   fprintf(stderr, "There are %u documents currently alive:\n", set.size());
   for (blink::Document* document : set)
     fprintf(stderr, "- Document %p URL: %s\n", document,
             document->Url().GetString().Utf8().data());
 }
 #endif