Add UseCounter to see how often the "allowfullscreen" attribute is modified.

third_party/WebKit/Source/core/html/HTMLIFrameElement.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Simon Hausmann (hausmann@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2006, 2008, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2009 Ericsson AB. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
@@ skipping 107 matching lines @@
     m_referrerPolicy = ReferrerPolicyDefault;
     if (!value.isNull()) {
       SecurityPolicy::referrerPolicyFromString(
           value, SupportReferrerPolicyLegacyKeywords, &m_referrerPolicy);
       UseCounter::count(document(),
                         UseCounter::HTMLIFrameElementReferrerPolicyAttribute);
     }
   } else if (name == allowfullscreenAttr) {
     bool oldAllowFullscreen = m_allowFullscreen;
     m_allowFullscreen = !value.isNull();
-    if (m_allowFullscreen != oldAllowFullscreen)
+    if (m_allowFullscreen != oldAllowFullscreen) {
+      // TODO(iclelland): Remove this use counter when the allowfullscreen
+      // attribute state is snapshotted on document creation. crbug.com/682282
+      if (m_allowFullscreen && contentFrame()) {
+        UseCounter::count(
+            document(),
+            UseCounter::
+                HTMLIFrameElementAllowfullscreenAttributeSetAfterContentLoad);
+      }
       frameOwnerPropertiesChanged();
+    }
   } else if (name == allowpaymentrequestAttr) {
     bool oldAllowPaymentRequest = m_allowPaymentRequest;
     m_allowPaymentRequest = !value.isNull();
     if (m_allowPaymentRequest != oldAllowPaymentRequest)
       frameOwnerPropertiesChanged();
   } else if (name == permissionsAttr) {
     if (initializePermissionsAttribute())
       m_permissions->setValue(value);
   } else if (RuntimeEnabledFeatures::embedderCSPEnforcementEnabled() &&
              name == cspAttr) {
@@ skipping 78 matching lines @@
 bool HTMLIFrameElement::initializePermissionsAttribute() {
   if (!RuntimeEnabledFeatures::permissionDelegationEnabled())
     return false;
 
   if (!m_permissions)
     m_permissions = HTMLIFrameElementPermissions::create(this);
   return true;
 }
 
 }  // namespace blink