Add WebFont.HadBlankText and WebFont.BlankTextShownTime histograms

The Blink-side change is here: https://codereview.chromium.org/180273020/

BUG=345839
NOTRY=true

Review URL: https://codereview.chromium.org/183743016

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255225 0039d316-1c4b-4281-b951-d872f2087c98

tools/metrics/histograms/histograms.xml

@@ -24047,6 +24047,14 @@ other types of suffix sets.
   </summary>
 </histogram>
 
+<histogram name="WebFont.BlankTextShownTime" units="milliseconds">
+  <summary>
+    A histogram tracking the time we spent showing blank text because a web font
+    wasn't available by the time we needed it. Measured once per @font-face that
+    ended up showing blank text.
+  </summary>
+</histogram>
+
 <histogram name="WebFont.CacheHit" enum="WebFontCacheHit">
   <summary>
     Recorded upon web fonts load. Counts the number of times web font is loaded
@@ -24097,6 +24105,14 @@ other types of suffix sets.
   </summary>
 </histogram>
 
+<histogram name="WebFont.HadBlankText" enum="BooleanHadBlankText">
+  <summary>
+    This metrics is logged when a page that use web fonts is loaded. The value
+    is whether we had to wait on at least one web font and ended up showing
+    blank text, or not.
+  </summary>
+</histogram>
+
 <histogram name="WebFont.LayoutLatency" units="milliseconds">
   <obsolete>
     Renamed to WebFont.StyleRecalcToDownloadLatency for clarity.
@@ -25091,6 +25107,11 @@ other types of suffix sets.
   <int value="1" label="Force Disabled"/>
 </enum>
 
+<enum name="BooleanHadBlankText" type="int">
+  <int value="0" label="Did not have blank text"/>
+  <int value="1" label="Had blank text"/>
+</enum>
+
 <enum name="BooleanHasCrc" type="int">
   <int value="0" label="No CRC"/>
   <int value="1" label="Has CRC"/>