Add histograms for deletion of session cookies on startup.

The new histograms measure the performance of session cookie deletion.

BUG=485694

Review URL: https://codereview.chromium.org/1131413004

Cr-Commit-Position: refs/heads/master@{#329529}

content/browser/net/sqlite_persistent_cookie_store.cc

@@ -1305,8 +1305,14 @@ void SQLitePersistentCookieStore::Backend::SetForceKeepSessionState() {
 
 void SQLitePersistentCookieStore::Backend::DeleteSessionCookiesOnStartup() {
   DCHECK(background_task_runner_->RunsTasksOnCurrentThread());
+  base::Time start_time = base::Time::Now();
   if (!db_->Execute("DELETE FROM cookies WHERE persistent != 1"))
     LOG(WARNING) << "Unable to delete session cookies.";
+
+  UMA_HISTOGRAM_TIMES("Cookie.Startup.TimeSpentDeletingCookies",
+                      base::Time::Now() - start_time);
+  UMA_HISTOGRAM_COUNTS("Cookie.Startup.NumberOfCookiesDeleted",
+                       db_->GetLastChangeCount());
 }
 
 void SQLitePersistentCookieStore::Backend::PostBackgroundTask(

tools/metrics/histograms/histograms.xml

@@ -3838,6 +3838,24 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Cookie.Startup.NumberOfCookiesDeleted">
+  <owner>erikchen@chromium.org</owner>
+  <summary>
+    The number of session cookies deleted on startup. This metric is emitted
+    even if no cookies are deleted. This metric is not emitted if session
+    cookies are restored, rather than deleted on startup.
+  </summary>
+</histogram>
+
+<histogram name="Cookie.Startup.TimeSpentDeletingCookies" units="ms">
+  <owner>erikchen@chromium.org</owner>
+  <summary>
+    The amount of time required to delete session cookies on startup. This
+    metric is emitted even if no cookies are deleted. This metric is not emitted
+    if session cookies are restored, rather than deleted on startup.
+  </summary>
+</histogram>
+
 <histogram name="Cookie.TimeBlockedOnLoad" units="ms">
   <owner>erikwright@chromium.org</owner>
   <summary>