Add DocumentTiming metrics for "time to first text paint"

Source/platform/fonts/Font.h

Index: Source/platform/fonts/Font.h
diff --git a/Source/platform/fonts/Font.h b/Source/platform/fonts/Font.h
index 8885e647041d92a5ce7e8996f004c7a00778a3f9..db49a7a649a1499f85fe445cc04b9f6d6302114e 100644
--- a/Source/platform/fonts/Font.h
+++ b/Source/platform/fonts/Font.h
@@ -137,6 +137,7 @@ private:
     FloatRect selectionRectForComplexText(const TextRun&, const FloatPoint&, int h, int from, int to) const;
 
     bool computeCanShapeWordByWord() const;
+    void reportFirstTextPaint() const;
 
     friend struct SimpleShaper;
 
@@ -158,6 +159,7 @@ private:
     mutable RefPtr<FontFallbackList> m_fontFallbackList;
     mutable unsigned m_canShapeWordByWord : 1;
     mutable unsigned m_shapeWordByWordComputed : 1;
+    mutable unsigned m_firstPaintReported : 1;
 };
 
 inline Font::~Font()