Add DocumentTiming metrics for "time to first text paint"

Source/platform/fonts/Font.cpp

Index: Source/platform/fonts/Font.cpp
diff --git a/Source/platform/fonts/Font.cpp b/Source/platform/fonts/Font.cpp
index aa087069dcd1202865b9e157dacb175a2be0e576..7c19b34c86a0a57a12b43038013548d1eddc9bcf 100644
--- a/Source/platform/fonts/Font.cpp
+++ b/Source/platform/fonts/Font.cpp
@@ -62,6 +62,7 @@ Font::Font(const FontDescription& fd)
     : m_fontDescription(fd)
     , m_canShapeWordByWord(0)
     , m_shapeWordByWordComputed(0)
+    , m_firstPaintReported(0)
 {
 }
 
@@ -70,6 +71,7 @@ Font::Font(const Font& other)
     , m_fontFallbackList(other.m_fontFallbackList)
     , m_canShapeWordByWord(0)
     , m_shapeWordByWordComputed(0)
+    , m_firstPaintReported(0)
 {
 }
 
@@ -79,6 +81,7 @@ Font& Font::operator=(const Font& other)
     m_fontFallbackList = other.m_fontFallbackList;
     m_canShapeWordByWord = other.m_canShapeWordByWord;
     m_shapeWordByWordComputed = other.m_shapeWordByWordComputed;
+    m_firstPaintReported = other.m_firstPaintReported;
     return *this;
 }
 
@@ -143,6 +146,8 @@ void Font::drawText(SkCanvas* canvas, const TextRunPaintInfo& runInfo,
     if (shouldSkipDrawing())
         return;
 
+    reportFirstTextPaint();
+
     if (runInfo.cachedTextBlob && runInfo.cachedTextBlob->get()) {
         // we have a pre-cached blob -- happy joy!
         drawTextBlob(canvas, paint, runInfo.cachedTextBlob->get(), point.data());
@@ -163,6 +168,8 @@ void Font::drawBidiText(SkCanvas* canvas, const TextRunPaintInfo& runInfo, const
     if (shouldSkipDrawing() && customFontNotReadyAction == DoNotPaintIfFontNotReady)
         return;
 
+    reportFirstTextPaint();
+
     // sub-run painting is not supported for Bidi text.
     const TextRun& run = runInfo.run;
     ASSERT((runInfo.from == 0) && (runInfo.to == run.length()));
@@ -201,6 +208,17 @@ void Font::drawBidiText(SkCanvas* canvas, const TextRunPaintInfo& runInfo, const
     bidiRuns.deleteRuns();
 }
 
+void Font::reportFirstTextPaint() const
+{
+    if (m_firstPaintReported || !fontSelector())
+        return;
+    m_firstPaintReported = true;
+
+    if (m_fontFallbackList->hasCustomFont())
+        fontSelector()->reportFirstCustomFontText();
+    fontSelector()->reportFirstNonBlankText();
+}
+
 void Font::drawEmphasisMarks(SkCanvas* canvas, const TextRunPaintInfo& runInfo, const AtomicString& mark, const FloatPoint& point, float deviceScaleFactor, const SkPaint& paint) const
 {
     if (shouldSkipDrawing())