Add DocumentTiming metrics for "time to first text paint"

Source/core/css/CSSFontSelector.h

 /*
  * Copyright (C) 2007, 2008, 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 58 matching lines @@
 #if !ENABLE(OILPAN)
     void unregisterForInvalidationCallbacks(CSSFontSelectorClient*);
 #endif
 
     Document* document() const { return m_document; }
     FontFaceCache* fontFaceCache() { return &m_fontFaceCache; }
     FontLoader* fontLoader() { return m_fontLoader.get(); }
 
     const GenericFontFamilySettings& genericFontFamilySettings() const { return m_genericFontFamilySettings; }
     void updateGenericFontFamilySettings(Document&);
+    void reportFirstNonBlankText(bool isCustomFont) override;
 
     DECLARE_VIRTUAL_TRACE();
 
 protected:
     explicit CSSFontSelector(Document*);
 
     void dispatchInvalidationCallbacks();
 
 private:
     // FIXME: Oilpan: Ideally this should just be a traced Member but that will
     // currently leak because ComputedStyle and its data are not on the heap.
     // See crbug.com/383860 for details.
     RawPtrWillBeWeakMember<Document> m_document;
     // FIXME: Move to Document or StyleEngine.
     FontFaceCache m_fontFaceCache;
     WillBeHeapHashSet<RawPtrWillBeWeakMember<CSSFontSelectorClient>> m_clients;
 
     RefPtrWillBeMember<FontLoader> m_fontLoader;
     GenericFontFamilySettings m_genericFontFamilySettings;
 };
 
 } // namespace blink
 
 #endif // CSSFontSelector_h