Add DocumentTiming metrics for "time to first text paint"

Source/platform/fonts/FontSelector.h

Index: Source/platform/fonts/FontSelector.h
diff --git a/Source/platform/fonts/FontSelector.h b/Source/platform/fonts/FontSelector.h
index 8bd8c8686de4f045eb9d031abd477d5483aa98a5..fbe8eca2b0e128b89b15ccdc8136c1c15fa201a7 100644
--- a/Source/platform/fonts/FontSelector.h
+++ b/Source/platform/fonts/FontSelector.h
@@ -42,6 +42,7 @@ public:
     virtual ~FontSelector() { }
     virtual PassRefPtr<FontData> getFontData(const FontDescription&, const AtomicString& familyName) = 0;
     virtual void willUseFontData(const FontDescription&, const AtomicString& familyName, UChar32) = 0;
+    virtual void reportFirstNonBlankText(bool isCustomFont) { }
 
     virtual unsigned version() const = 0;
 };