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 2f8df067687647edf40dbeec32e4f83d2dddac51..786ee75fce4bc225e19408998c66043bfb3a406c 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -13589,6 +13589,26 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <summary>Number of calls to extension functions.</summary>
 </histogram>
 
+<histogram name="Extensions.Functions.SynchronousExecutionTime" units="ms">

[Devlin, 2016/05/03 00:43:49]

Mark: I see other places that use TimeTicks/TimeDelta (based on us) using
UMA_HISTOGRAM_TIMES() with units ms, but it's unclear from the documentation if
it should be ms or us as the units.  Can you confirm?  (I briefly tried to dive
into the UMA_HISTOGRAM_TIMES macro, but got quickly lost.)

[Mark P, 2016/05/03 18:30:39]

All the UMA_HISTOGRAM_TIMES macros use milliseconds.
It's true that TimeTicks/TimeDelta can give you more accuracy, but the
histograms macros don't use or record it.
Let me know if you saw something where you think someone is expecting a
histogram to be providing microsecond-based measurements.  That may be a bug.

+  <owner>rdevlin.cronin@chromium.org</owner>
+  <summary>
+    The synchronous amount of time an extension function takes. Note that many
+    extension functions run asynchronously; see also
+    Extensions.Functions.TotalExecutionTime.
+  </summary>
+</histogram>
+
+<histogram name="Extensions.Functions.TotalExecutionTime" units="ms">
+  <owner>rdevlin.cronin@chromium.org</owner>
+  <summary>
+    The total amount of time it takes to execute an extension function from the
+    time the extension function is called to the time the function responds.
+    Note that since some extension functions are inherently slow (anything that
+    requires user interaction, for instance), this is not a definitive source
+    for function performance.
+  </summary>
+</histogram>
+
 <histogram name="Extensions.GetUserDataTempDir" enum="GetUserDataTempDirResult">
   <owner>Please list the metric's owners. Add more owner tags as needed.</owner>
   <summary>