Add DocumentTiming metrics for "time to first text paint"

Source/platform/fonts/FontFallbackList.cpp

Index: Source/platform/fonts/FontFallbackList.cpp
diff --git a/Source/platform/fonts/FontFallbackList.cpp b/Source/platform/fonts/FontFallbackList.cpp
index 284f6124f883c5305bc5d99b66b7b8ddc7310688..eddf6f58164f2cfa9db1c8ecb4d1aae3670a39f3 100644
--- a/Source/platform/fonts/FontFallbackList.cpp
+++ b/Source/platform/fonts/FontFallbackList.cpp
@@ -46,6 +46,7 @@ FontFallbackList::FontFallbackList()
     , m_familyIndex(0)
     , m_generation(FontCache::fontCache()->generation())
     , m_hasLoadingFallback(false)
+    , m_hasCustomFont(false)
 {
 }
 
@@ -58,6 +59,7 @@ void FontFallbackList::invalidate(PassRefPtrWillBeRawPtr<FontSelector> fontSelec
     m_cachedPrimarySimpleFontData = 0;
     m_familyIndex = 0;
     m_hasLoadingFallback = false;
+    m_hasCustomFont = false;
     if (m_fontSelector != fontSelector)
         m_fontSelector = fontSelector;
     m_fontSelectorVersion = m_fontSelector ? m_fontSelector->version() : 0;
@@ -211,6 +213,8 @@ const FontData* FontFallbackList::fontDataAt(const FontDescription& fontDescript
         m_fontList.append(result);
         if (result->isLoadingFallback())
             m_hasLoadingFallback = true;
+        if (result->isCustomFont())

[drott, 2015/09/16 07:26:20]

Note that recording the m_hasCustomFont flag here, when fontDataAt(a) is called
with an index that points to a custom font, is not 100% equivalent to the
accessed font getting used for drawing. This might make the intended UMA data
you're trying to record inaccurate.

Instead, you might want to check whether SimpleFontData->isCustomFont() in
drawGlyphBuffer() - however, as I mentioned before, somehow it needs to be
ensure that we do this really only once per pageload and avoid introducing
additional cycles into drawGlyphBuffer.

[Kunihiko Sakamoto, 2015/09/17 08:36:10]

I see your point, but I think this condition is appropriate for our purpose.

Rather than actual font used for drawing, I'm interested in distinguishing if
the text paint is blocked by font load or not. If a webfont is loaded and then
fallbacked to local font (e.g. because loaded font did not have a glyph), we'd
like to count it as "custom font text".

+            m_hasCustomFont = true;
     }
     return result.get();
 }