Only record the execution time during startup.

components/history/core/browser/top_sites_backend.cc

Index: components/history/core/browser/top_sites_backend.cc
diff --git a/components/history/core/browser/top_sites_backend.cc b/components/history/core/browser/top_sites_backend.cc
index 2383c1c9d213510dc217bc28465667d05dda2392..a3fe1d9aeb6ad13c308a808f552b3d7fdec8f7fe 100644
--- a/components/history/core/browser/top_sites_backend.cc
+++ b/components/history/core/browser/top_sites_backend.cc
@@ -47,10 +47,11 @@ void TopSitesBackend::GetMostVisitedThumbnails(
       base::Bind(callback, thumbnails));
 }
 
-void TopSitesBackend::UpdateTopSites(const TopSitesDelta& delta) {
+void TopSitesBackend::UpdateTopSites(const TopSitesDelta& delta, bool startup) {
   db_task_runner_->PostTask(
       FROM_HERE,
-      base::Bind(&TopSitesBackend::UpdateTopSitesOnDBThread, this, delta));
+      base::Bind(&TopSitesBackend::UpdateTopSitesOnDBThread, this, delta,
+                 startup));
 }
 
 void TopSitesBackend::SetPageThumbnail(const MostVisitedURL& url,
@@ -101,7 +102,8 @@ void TopSitesBackend::GetMostVisitedThumbnailsOnDBThread(
   }
 }
 
-void TopSitesBackend::UpdateTopSitesOnDBThread(const TopSitesDelta& delta) {
+void TopSitesBackend::UpdateTopSitesOnDBThread(const TopSitesDelta& delta,
+                                               bool startup) {
   TRACE_EVENT0("startup", "history::TopSitesBackend::UpdateTopSitesOnDBThread");
 
   if (!db_)
@@ -120,8 +122,10 @@ void TopSitesBackend::UpdateTopSitesOnDBThread(const TopSitesDelta& delta) {
   for (size_t i = 0; i < delta.moved.size(); ++i)
     db_->UpdatePageRank(delta.moved[i].url, delta.moved[i].rank);
 
-  UMA_HISTOGRAM_TIMES("History.UpdateTopSitesOnDBThreadTime",
-                      base::TimeTicks::Now() - begin_time);
+  if (startup) {
+    UMA_HISTOGRAM_TIMES("History.UpdateTopSitesOnDBThread_Startup_Time",

[sky, 2015/03/31 23:58:49]

Didn't you want to put the size of delta in the name?

[yao, 2015/04/01 21:43:00]

The delta size is recorded in TopSitesImpl::SetTopSites, so that the delta size
is also recorded when it's 0. This way we know what percentage of times delta is
zero/not zero. 

+                        base::TimeTicks::Now() - begin_time);
+  }
 }
 
 void TopSitesBackend::SetPageThumbnailOnDBThread(const MostVisitedURL& url,