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