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 1043e5631c3993d0e32a18ba3388cdb14f3014aa..802584bc120d2aebe5b8ec681ebfe44a191451d7 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -63311,6 +63311,14 @@ 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. Note that a
+    stylesheet which uses all of its rules will log counts in every bucket.

[Ilya Sherman, 2016/12/01 00:48:21]

Please document precisely when this metric is recorded -- when the page is fully
parsed?

[Ilya Sherman, 2016/12/01 00:48:21]

Hmm, why structure this to record to every bucket?  It seems like interpreting
the data would be easier if each parse recorded to just a single bucket.

[Charlie Harrison, 2016/12/01 02:43:51]

Yeah this would be ideal, but it is very hard to determine when to actually
record the metric. Ideally we would do this in the destructor, but fast shutdown
makes this very unreliable.

For a bit of context: we share StyleSheetContexts, often between Documents, so
if we record based on Document lifecycle events we'll sometimes get inaccurate
data.

[Charlie Harrison, 2016/12/01 02:43:51]

Done. We log as the rules are parsed.

[Ilya Sherman, 2016/12/01 05:10:54]

Ah, okay, that makes sense.  Fair enough -- thanks!

+  </summary>
+</histogram>
+
 <histogram name="Style.UpdateTime" units="us">
   <owner>csharrison@chromium.org</owner>
   <summary>Microseconds spent in Document::updateStyle.</summary>
@@ -91192,6 +91200,16 @@ 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"/>