Add histograms for deletion of session cookies on startup.

content/browser/net/sqlite_persistent_cookie_store.cc

Index: content/browser/net/sqlite_persistent_cookie_store.cc
diff --git a/content/browser/net/sqlite_persistent_cookie_store.cc b/content/browser/net/sqlite_persistent_cookie_store.cc
index 6b2ff26ec3703f05d83b3d892ac16b6468a35700..00fb10e02dbe84bcc2e1e8d7a8437e808e0323b5 100644
--- a/content/browser/net/sqlite_persistent_cookie_store.cc
+++ b/content/browser/net/sqlite_persistent_cookie_store.cc
@@ -701,8 +701,27 @@ bool SQLitePersistentCookieStore::Backend::InitializeDatabase() {
 
   initialized_ = true;
 
-  if (!restore_old_session_cookies_)
+  int deleted_cookies_count = 0;

[erikwright (departed), 2015/05/12 14:30:43]

Assuming you agree with my comment below, move count declaration to line 714 and
inline the calculation of elapsed time on line 712.

[erikchen, 2015/05/12 18:50:34]

I removed the variables elapsed_time and deleted_cookies_count.

+  base::TimeDelta elapsed_time;
+  if (!restore_old_session_cookies_) {
+    base::Time start_time = base::Time::Now();
     DeleteSessionCookiesOnStartup();
+
+    elapsed_time = base::Time::Now() - start_time;
+    UMA_HISTOGRAM_TIMES("Cookie.Startup.TimeSpentDeletingCookies",
+                        elapsed_time);
+
+    deleted_cookies_count = db_->GetLastChangeCount();
+    UMA_HISTOGRAM_COUNTS("Cookie.Startup.NumberOfCookiesDeleted",
+                         deleted_cookies_count);
+  }
+
+  if (deleted_cookies_count > 0) {

[erikwright (departed), 2015/05/12 14:30:43]

I'm not convinced that this is useful. In any case, it can be generated via
dremel (query for UMA uploads containg both the TimeSpent.. and
NumberOfCookies.. and do the division there).

What would be more interesting would be to do a scatterplot of those two
numbers, to see if there is a clear relationship between them.

[erikchen, 2015/05/12 18:50:34]

I wasn't aware that this was easy to do with dremel. I'm guessing that the
scatterplot you're referring to is also done via dremel? 

I've removed this histogram.

+    base::TimeDelta avg_deletion_time = elapsed_time / deleted_cookies_count;
+    UMA_HISTOGRAM_TIMES("Cookie.Startup.AverageTimeSpentDeletingOneCookie",
+                        avg_deletion_time);
+  }
+
   return true;
 }