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 500c47d0ae272e6042d4d995f4d05deca146b85c..e2b3a324ccee93273d4993bd9894bc50a5f10c69 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -11362,7 +11362,31 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="History.UpdateTopSitesOnDBThread_Startup_DeltaSize">

[jwd, 2015/03/30 16:06:48]

Is there a unit for this?

[yao, 2015/03/30 22:16:18]

Just integer. 

[jwd, 2015/03/31 19:26:16]

I see, can you add to the description to make it clear that it is a delta of the
number of top sites. Sizes makes me think too much of storage size (which made
sense to me since it's talking about a DB).

+  <owner>yiyaoliu@chromium.org</owner>
+  <summary>
+    The delta size between cached top sites and up-to-date top sites when
+    function TopSitesImpl::SetTopSites is firstly called at startup.
+  </summary>
+</histogram>
+
+<histogram name="History.UpdateTopSitesOnDBThread_Startup_Time"
+    units="milliseconds">
+  <owner>yiyaoliu@chromium.org</owner>
+  <summary>
+    The amount of time for function
+    history::TopSitesBackend::UpdateTopSitesOnDBThread to execute when this
+    function is 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" units="milliseconds">
+  <obsolete>
+    Deprecated because only the execution time at startup is of interest. See
+    histogram History.UpdateTopSitesOnDBThread_Startup_Time.
+  </obsolete>
   <owner>yiyaoliu@chromium.org</owner>
   <summary>
     The amount of time for function