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..473f6faf2f06a4de941ca68333f0dd867d024959 100644
--- a/Source/core/css/CSSFontSelector.cpp
+++ b/Source/core/css/CSSFontSelector.cpp
@@ -47,6 +47,8 @@ CSSFontSelector::CSSFontSelector(Document* document)
     : m_document(document)
     , m_fontLoader(FontLoader::create(this, document))
     , m_genericFontFamilySettings(document->frame()->settings()->genericFontFamilySettings())
+    , m_firstNonBlankTextReported(false)
+    , m_firstCustomFontTextReported(false)
 {
     // FIXME: An old comment used to say there was no need to hold a reference to m_document
     // because "we are guaranteed to be destroyed before the document". But there does not
@@ -128,6 +130,20 @@ static AtomicString familyNameFromSettings(const GenericFontFamilySettings& sett
     return emptyAtom;
 }
 
+void CSSFontSelector::reportFirstNonBlankText(bool isCustomFont)
+{
+    if (isCustomFont) {
+        if (m_firstCustomFontTextReported)

[kinuko, 2015/09/14 14:14:33]

Looks like common pattern is doing something like following, should we follow?

if (!documentTiming.firstFooTiming())
  documentTiming.markFirstFooTiming();

Also I wonder if we should report these via document class, so that it's easier
to track all document related timings.

[Kunihiko Sakamoto, 2015/09/15 05:11:01]

Done.

+            return;
+        m_document->mutableTiming().markFirstCustomFontText();
+        m_firstCustomFontTextReported = true;
+    }
+    if (m_firstNonBlankTextReported)
+        return;
+    m_document->mutableTiming().markFirstNonBlankText();
+    m_firstNonBlankTextReported = true;
+}
+
 PassRefPtr<FontData> CSSFontSelector::getFontData(const FontDescription& fontDescription, const AtomicString& familyName)
 {
     if (CSSSegmentedFontFace* face = m_fontFaceCache.get(fontDescription, familyName))