Experiment with blocking script inside fragment-parser-inserted `<iframe srcdoc>`.

third_party/WebKit/Source/core/dom/ScriptLoader.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) 2003, 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights
  * reserved.
  * Copyright (C) 2008 Nikolas Zimmermann <zimmermann@kde.org>
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
@@ skipping 62 matching lines @@
       m_willBeParserExecuted(false),
       m_readyToBeParserExecuted(false),
       m_willExecuteWhenDocumentFinishedParsing(false),
       m_forceAsync(!parserInserted),
       m_createdDuringDocumentWrite(createdDuringDocumentWrite),
       m_asyncExecType(ScriptRunner::None),
       m_documentWriteIntervention(
           DocumentWriteIntervention::DocumentWriteInterventionNone) {
   DCHECK(m_element);
   if (parserInserted && element->document().scriptableDocumentParser() &&
-      !element->document().isInDocumentWrite())
+      !element->document().isInDocumentWrite()) {
     m_startLineNumber =
         element->document().scriptableDocumentParser()->lineNumber();
+  }
+
+  // Experiment based on https://github.com/whatwg/html/issues/2300.
+  if (!alreadyStarted && element->document().isFragmentParserCreatedSrcdoc()) {
+    UseCounter::count(element->document(),
+                      UseCounter::ScriptInsideFragmentParserInsertedSrcdoc);
+    if (RuntimeEnabledFeatures::
+            scriptInsideParserInsertedSrcdocIsAlreadyStartedEnabled()) {
+      m_alreadyStarted = true;
+    }
+  }
 }
 
 ScriptLoader::~ScriptLoader() {}
 
 DEFINE_TRACE(ScriptLoader) {
   visitor->trace(m_element);
   visitor->trace(m_resource);
   visitor->trace(m_pendingScript);
   PendingScriptClient::trace(visitor);
 }
@@ skipping 546 matching lines @@
   if (isHTMLScriptLoader(element))
     return toHTMLScriptElement(element)->loader();
 
   if (isSVGScriptLoader(element))
     return toSVGScriptElement(element)->loader();
 
   return 0;
 }
 
 }  // namespace blink