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

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

Index: third_party/WebKit/Source/core/dom/ScriptLoader.cpp
diff --git a/third_party/WebKit/Source/core/dom/ScriptLoader.cpp b/third_party/WebKit/Source/core/dom/ScriptLoader.cpp
index e6a6769683aa230c961fb234b0c5fbd15492a219..7b8ee676ca887998fb45bb6605224c067e57711b 100644
--- a/third_party/WebKit/Source/core/dom/ScriptLoader.cpp
+++ b/third_party/WebKit/Source/core/dom/ScriptLoader.cpp
@@ -80,9 +80,20 @@ ScriptLoader::ScriptLoader(Element* element,
           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() {}