Add DocumentTiming metrics for "time to first text paint"

Source/core/dom/Document.cpp

Index: Source/core/dom/Document.cpp
diff --git a/Source/core/dom/Document.cpp b/Source/core/dom/Document.cpp
index 9d708509afd13473ba6435c070254941081a3ec8..b573b66f22b8f645b4b70ee12705183a97015211 100644
--- a/Source/core/dom/Document.cpp
+++ b/Source/core/dom/Document.cpp
@@ -1918,6 +1918,18 @@ void Document::layoutUpdated()
     }
 }
 
+void Document::markFirstCustomFontText()
+{
+    if (!m_documentTiming.firstCustomFontText())
+        m_documentTiming.markFirstCustomFontText();
+}
+
+void Document::markFirstNonBlankText()
+{
+    if (!m_documentTiming.firstNonBlankText())
+        m_documentTiming.markFirstNonBlankText();
+}
+
 void Document::setNeedsFocusedElementCheck()
 {
     setNeedsStyleRecalc(LocalStyleChange, StyleChangeReasonForTracing::createWithExtraData(StyleChangeReason::PseudoClass, StyleChangeExtraData::Focus));