Add DocumentTiming metrics for "time to first text paint"

Source/core/css/CSSFontSelector.cpp

Index: Source/core/css/CSSFontSelector.cpp
diff --git a/Source/core/css/CSSFontSelector.cpp b/Source/core/css/CSSFontSelector.cpp
index d984785f0638362ca88bdce4deb68553a3cea4a3..92f1e33806a86caada1c9122d1f074e9def0d4f1 100644
--- a/Source/core/css/CSSFontSelector.cpp
+++ b/Source/core/css/CSSFontSelector.cpp
@@ -128,6 +128,13 @@ static AtomicString familyNameFromSettings(const GenericFontFamilySettings& sett
     return emptyAtom;
 }
 
+void CSSFontSelector::reportFirstNonBlankText(bool isCustomFont)
+{
+    if (isCustomFont)
+        m_document->markFirstCustomFontText();
+    m_document->markFirstNonBlankText();
+}
+
 PassRefPtr<FontData> CSSFontSelector::getFontData(const FontDescription& fontDescription, const AtomicString& familyName)
 {
     if (CSSSegmentedFontFace* face = m_fontFaceCache.get(fontDescription, familyName))