Record "time to first text paint" UMA metrics in PageLoadHistograms

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index d847369e9a1a2be0e47864f0c856ddf7c3cabfcd..9b2553180d1a272d39366b2540b6fd1c5c43fdd0 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -32213,6 +32213,24 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="PLT.BeginToFirstCustomFontText" units="milliseconds">
+  <owner>ksakamoto@chromium.org</owner>
+  <summary>
+    Time from &quot;begin&quot; to &quot;first custom font text paint.&quot;
+    &quot;First custom font text paint&quot;== time when first paint operation
+    for visible text with custom font (defined by @font-face) was performed.
+  </summary>
+</histogram>
+
+<histogram name="PLT.BeginToFirstNonBlankText" units="milliseconds">
+  <owner>ksakamoto@chromium.org</owner>
+  <summary>
+    Time from &quot;begin&quot; to &quot;first non-blank text paint.&quot;
+    &quot;First non-blank text paint&quot; == time when first paint operation
+    for visible text was performed.
+  </summary>
+</histogram>
+
 <histogram name="PLT.BeginToFirstPaint" units="milliseconds">
   <owner>pmeenan@chromium.org</owner>
   <summary>