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..42fd66b45c54bf28928e4abcb59303af18b79f6a 100644
--- a/Source/core/css/CSSFontSelector.h
+++ b/Source/core/css/CSSFontSelector.h
@@ -50,10 +50,13 @@ public:
     }
     ~CSSFontSelector() override;
 
+    // FontSelector implementation
     unsigned version() const override { return m_fontFaceCache.version(); }
-
     PassRefPtr<FontData> getFontData(const FontDescription&, const AtomicString&) override;
     void willUseFontData(const FontDescription&, const AtomicString& family, UChar32) override;
+    void reportFirstCustomFontText() override;
+    void reportFirstNonBlankText() override;
+
     bool isPlatformFontAvailable(const FontDescription&, const AtomicString& family);
 
 #if !ENABLE(OILPAN)