Only record the execution time during startup.

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 bc7a43e7e94c75b84eaca3097f58a92f08b2a715..14b74d1aca8115636c8f81ba888e6a6636e0418b 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -11358,9 +11358,19 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <owner>yiyaoliu@chromium.org</owner>
   <summary>
     The amount of time for function
-    history::TopSitesBackend::UpdateTopSitesOnDBThread to execute. Excludes the
-    case where local TopSitesDatabase db_ is unavailable, i.e. where the update
-    doesn't really happen.
+    history::TopSitesBackend::UpdateTopSitesOnDBThread to execute when this

[rkaplow, 2015/03/27 02:33:28]

I would put this into a different histogram since you're changing what this is
measuring.

[yao, 2015/03/27 18:39:45]

Deleted the old one. 

+    function is not called during startup. Excludes the case where local
+    TopSitesDatabase db_ is unavailable, i.e. where the update doesn't really
+    happen.
+  </summary>
+</histogram>
+
+<histogram name="History.UpdateTopSitesOnDBThreadTime.Startup"
+    units="milliseconds">
+  <owner>yiyaoliu@chromium.org</owner>
+  <summary>
+    Same as History.UpdateTopSitesOnDBThreadTime except that this histogram only
+    records it when the function is called during startup.
   </summary>
 </histogram>