Add UMA stats to track whitelist download latency.

tools/metrics/histograms/histograms.xml

 <!--
 Copyright 2013 The Chromium Authors. All rights reserved.
 Use of this source code is governed by a BSD-style license that can be
 found in the LICENSE file.
 -->
 
 <!--
 This file is used to generate a comprehensive list of Chrome histograms along
 with a detailed description for each histogram.
 
@@ skipping 141 matching lines @@
 </histogram>
 
 <histogram name="Autocheckout.WalletRequiredActions"
     enum="WalletRequiredActions">
   <summary>
     Measures the frequency of required user actions returned by the Google
     Online Wallet server.
   </summary>
 </histogram>
 
+<histogram name="Autocheckout.WhitelistDownloadDuration" units="ms">

[ahutter, 2013/04/16 17:56:54]

+ramankk, does this look right?

+  <summary>
+    Measures the duration for download Autocheckout whitelist file.

[ahutter, 2013/04/16 17:56:54]

Measures time taken to download the...

To be consistent with Raman's summaries.

[benquan, 2013/04/17 01:51:30]

Done.

+  </summary>
+</histogram>
+
+<histogram name="Autocheckout.WhitelistDownloadDuration.Failed" units="ms">

[jar (doing other things), 2013/04/16 18:53:14]

nit: better would be to define a <field_trial...> below with the two possible
groups "Failed" and "Succeeded" as well as a separator="".

This way you don't need to repeat the bulk of the prose 3 times.

[benquan, 2013/04/17 01:51:30]

Can you please point me to a doc talks about how to use <field_trial...> and how
this xml block should look like? base/metrics/field_trial.h only tells me how to
modify the code, but not the xml file. And it seems like it is not trivial to
change the code, I'm not sure if it worth to trade the code change with a few
lines of xml, unless there are other benefits of using <field_trial>.

BTW, there are bunch of other status in this xml file similar to this, should we
convert them all together to make them consistent?

[Ilya Sherman, 2013/04/17 05:01:48]

Take a look at the top of this file, namely lines 26 through 49:  Jim's
suggestion does not require any changes to code outside of histograms.xml (and
the <fieldtrial> element is rather misleadingly named at this point).  That
said, for such short histogram descriptions, IMO it's nicer to just do what
you've done and write all of the histogram names out explicitly, because it's
easier to see what histograms, exactly, are being logged that way.  Since Jim
and I disagree here, I'd recommend just going with whatever seems cleaner to
you, or whatever seems more consistent with the surrounding "code".

[benquan, 2013/04/17 05:48:18]

I will go with "consistency", Jim, please let me know if you strongly disagree.

+  <summary>
+    Measures the duration for download Autocheckout whitelist file in case the

[ahutter, 2013/04/16 17:56:54]

Ditto.

[benquan, 2013/04/17 01:51:30]

Done.

+    download was failed.
+  </summary>
+</histogram>
+
+<histogram name="Autocheckout.WhitelistDownloadDuration.Succeeded" units="ms">
+  <summary>
+    Measures the duration for download Autocheckout whitelist file in case the

[ahutter, 2013/04/16 17:56:54]

Ditto.

[benquan, 2013/04/17 01:51:30]

Done.

+    download was succeeded.
+  </summary>
+</histogram>
+
 <histogram name="Autofill.AddressSuggestionsCount">
   <summary>
     The number of address suggestions shown in the Autofill popup.
   </summary>
 </histogram>
 
 <histogram name="AutoFill.CCInfoBarAccepted">
   <obsolete>
     Deprecated as of 3/2011, replaced by Autofill.CreditCardInfoBar.
   </obsolete>
@@ skipping 5193 matching lines @@
 <fieldtrial name="SSLResumption">
   <group name="Resume_Handshake" label="Session Resumption"/>
   <group name="Full_Handshake" label="Full"/>
   <affected-histogram name="Net.SSL_Connection_Latency"/>
   <affected-histogram name="Net.SSL_Connection_Latency_Google"/>
 </fieldtrial>
 
 </fieldtrials>
 
 </histogram-configuration>