[LazyParseCSS] Add histogram for rule usage %

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 83a7d1c616cb49285c447c03d08a9874acef0dca..d569996cfbd89e8b2962911ece7e97e01be161de 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -63660,6 +63660,17 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Style.LazyUsage.Percent" enum="LazyCSSParseUsage">
+  <owner>csharrison@chromium.org</owner>
+  <summary>
+    Tracks % of lazy rules that ended up needing to be parsed. A sheet logs
+    counts into this histogram as it is parsed (i.e. as properties are parsed
+    lazily). Once a certain percent of rules have been parsed, we log a count
+    immediately. Note that this implies that a stylesheet which uses all of its
+    rules will log counts in every bucket.
+  </summary>
+</histogram>
+
 <histogram name="Style.UpdateTime" units="microseconds">
   <owner>csharrison@chromium.org</owner>
   <summary>Microseconds spent in Document::updateStyle.</summary>
@@ -91697,6 +91708,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="268959744" label="NEW_TASK | NEW_DOCUMENT"/>
 </enum>
 
+<enum name="LazyCSSParseUsage" type="int">
+  <int value="0" label="&gt;= 0%"/>
+  <int value="1" label="&gt; 10%"/>
+  <int value="2" label="&gt; 25%"/>
+  <int value="3" label="&gt; 50%"/>
+  <int value="4" label="&gt; 75%"/>
+  <int value="5" label="&gt; 90%"/>
+  <int value="6" label="= 100%"/>
+</enum>
+
 <enum name="LevelDBCorruptionRestoreValue" type="int">
   <int value="0" label="Database Delete Success"/>
   <int value="1" label="Database Delete Failure"/>