UMA for the Stale Previews timestamp

tools/metrics/histograms/enums.xml

Index: tools/metrics/histograms/enums.xml
diff --git a/tools/metrics/histograms/enums.xml b/tools/metrics/histograms/enums.xml
index 5633395a7c7df6f4d71cbc84673a7cd55b02d2a2..0c4afb5bb380065412e0cfe979fa7d8296760f8c 100644
--- a/tools/metrics/histograms/enums.xml
+++ b/tools/metrics/histograms/enums.xml
@@ -29776,6 +29776,16 @@ from previous Chrome versions.
   <int value="5" label="Infobar dismissed by tab closure"/>
 </enum>
 
+<enum name="PreviewsInfoBarTimestamp">
+  <int value="0" label="Timestamp shown"/>
+  <int value="1" label="Timestamp not shown because the preview was not stale"/>
+  <int value="2"
+      label="Timestamp not shown because the staleness was negative."/>
+  <int value="3"
+      label="Timestamp not shown because the staleness was greater than the
+             max"/>
+</enum>
+
 <enum name="PreviewsUserOptedOut">
   <int value="0" label="The user did not choose to reload the full page."/>
   <int value="1" label="The user chose to reload the full page."/>