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 35df594736a3029fe839c4785f25b0891bc1f653..9b7996ccf95288d80e8780635b0b3abd67327b71 100644
--- a/tools/metrics/histograms/enums.xml
+++ b/tools/metrics/histograms/enums.xml
@@ -29746,6 +29746,19 @@ 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 Finch params were not available"/>
+    <int value="3"
+        label="Timestamp not shown because the staleness was negative."/>
+    <int value="4"
+        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."/>