Add DocumentTiming metrics for "time to first text paint"

Source/core/css/CSSFontSelector.h

Index: Source/core/css/CSSFontSelector.h
diff --git a/Source/core/css/CSSFontSelector.h b/Source/core/css/CSSFontSelector.h
index 1ef33994d7c1d8d2bd555c5f76fe134a8205a253..99c3f32977da4d5a34315efb8986d1824bf50704 100644
--- a/Source/core/css/CSSFontSelector.h
+++ b/Source/core/css/CSSFontSelector.h
@@ -76,6 +76,7 @@ public:
 
     const GenericFontFamilySettings& genericFontFamilySettings() const { return m_genericFontFamilySettings; }
     void updateGenericFontFamilySettings(Document&);
+    void reportFirstNonBlankText(bool isCustomFont) override;
 
     DECLARE_VIRTUAL_TRACE();
 
@@ -95,6 +96,8 @@ private:
 
     RefPtrWillBeMember<FontLoader> m_fontLoader;
     GenericFontFamilySettings m_genericFontFamilySettings;
+    bool m_firstNonBlankTextReported;
+    bool m_firstCustomFontTextReported;
 };
 
 } // namespace blink