Add a FOUC painting test.

Source/core/dom/Document.h

 /*
  * 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, 2010, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
@@ skipping 1017 matching lines @@
     NthIndexCache* nthIndexCache() const { return m_nthIndexCache; }
 
     bool isPrivilegedContext(String& errorMessage, const PrivilegeContextCheck = StandardPrivilegeCheck) const override;
 
     ClientHintsPreferences& clientHintsPreferences() { return m_clientHintsPreferences; }
 
     CanvasFontCache* canvasFontCache();
 
     // Used by unit tests so that all parsing will be main thread for
     // controlling parsing and chunking precisely.
-    static void setThreadedParsingEnabledForUnitTestsOnly(bool enabled);
-    static bool threadedParsingEnabledForUnitTestsOnly();
+    static void setThreadedParsingEnabledForTesting(bool);
+    static bool threadedParsingEnabledForTesting();
 
     void incrementNodeCount() { m_nodeCount++; }
     void decrementNodeCount()
     {
         ASSERT(m_nodeCount > 0);
         m_nodeCount--;
     }
     int nodeCount() const { return m_nodeCount; }
 
     using WeakDocumentSet = WillBeHeapHashSet<RawPtrWillBeWeakMember<Document>>;
@@ skipping 384 matching lines @@
 DEFINE_TYPE_CASTS(TreeScope, Document, document, true, true);
 
 } // namespace blink
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 CORE_EXPORT void showLiveDocumentInstances();
 #endif
 
 #endif // Document_h