Add DocumentTiming metrics for "time to first text paint"

Source/core/dom/DocumentTiming.h

Index: Source/core/dom/DocumentTiming.h
diff --git a/Source/core/dom/DocumentTiming.h b/Source/core/dom/DocumentTiming.h
index 034eb1aff6aa50f8b9542aa68f2ae0710c9cfe82..6cd1f390286f76c322bb8c3a49d14b1474193131 100644
--- a/Source/core/dom/DocumentTiming.h
+++ b/Source/core/dom/DocumentTiming.h
@@ -43,6 +43,8 @@ public:
     void markDomContentLoadedEventEnd();
     void markDomComplete();
     void markFirstLayout();
+    void markFirstNonBlankText();
+    void markFirstCustomFontText();
 
     double domLoading() const { return m_domLoading; }
     double domInteractive() const { return m_domInteractive; }
@@ -50,6 +52,8 @@ public:
     double domContentLoadedEventEnd() const { return m_domContentLoadedEventEnd; }
     double domComplete() const { return m_domComplete; }
     double firstLayout() const { return m_firstLayout; }
+    double firstNonBlankText() const { return m_firstNonBlankText; }
+    double firstCustomFontText() const { return m_firstCustomFontText; }
 
     DECLARE_TRACE();
 
@@ -62,6 +66,8 @@ private:
     double m_domContentLoadedEventEnd;
     double m_domComplete;
     double m_firstLayout;
+    double m_firstNonBlankText;
+    double m_firstCustomFontText;
 
     RawPtrWillBeMember<Document> m_document;
 };