Add the client for accessing Subresource Filter only list.

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 9658b2ee6db123ea65e1381698a0a5463d82d1bc..8d5a7a4fa9af5a1b85c61caeb5b6090ce53cedb8 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -67709,12 +67709,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 
 <histogram name="SubresourceFilter.PageLoad.RedirectChainLength" units="urls">
   <owner>melandory@chromium.org</owner>
+  <suffix name="SubresourceFilterOnly"/>

[jwd, 2017/03/07 22:31:17]

Remove these suffix tags, only the histogram_suffixes is needed.

[melandory, 2017/03/08 19:37:25]

Done.

   <summary>Total length of the server redirects during the navigation.</summary>

[jwd, 2017/03/07 22:31:17]

Can you reword the summary, either to "...server redirect chain during..." or
"Total number of server redirects..." 

[melandory, 2017/03/08 19:37:25]

Done.

 </histogram>
 
 <histogram name="SubresourceFilter.PageLoad.RedirectChainMatchPattern"
     enum="SubresourceFilterMatchPattern">
   <owner>melandory@chromium.org</owner>
+  <suffix name="SubresourceFilterOnly"/>
   <summary>
     For each main frame navigation, records a pattern that indicates which URLs
     in the redirect chain matched Safe Browsing blacklists, and which did not.
@@ -120582,6 +120584,13 @@ value.
   <affected-histogram name="WebRTC.Stun.SuccessPercent.UnknownNAT"/>
 </histogram_suffixes>
 
+<histogram_suffixes name="SubresourceFilterOnlyExperiment" separator=".">
+  <suffix name="SubresourceFilterOnly"/>
+  <affected-histogram name="SubresourceFilter.PageLoad.RedirectChainLength"/>
+  <affected-histogram
+      name="SubresourceFilter.PageLoad.RedirectChainMatchPattern"/>

[jwd, 2017/03/07 22:31:17]

Histogram suffixes have a hierarchical implication. Based on this, I'd assume
that SubresourceFilter.PageLoad.RedirectChainLength has the length for all the
cases, and SubresourceFilter.PageLoad.RedirectChainLength.SubresourceFilterOnly
has the length for a subset. From the code, it looks like this isn't the case.
I'd suggest creating a suffix for the non-SubresourceFilterOnly case, and either
having the base histograms capture the total values, or not log to the base
histogram anymore.

If you want to keep it as is please add a comment to both the affected
histograms making it clear that they are disjoint from the SubresourceFilterOnly
case.

[melandory, 2017/03/08 19:37:25]

I'm implementing suffixes in followup CL:
https://codereview.chromium.org/2733833002/

[jwd, 2017/03/08 22:59:26]

Yes, but that CL appears to have the same problem. My understanding of that code
is that you're using the base histograms
(SubresourceFilter.PageLoad.RedirectChainLength,
SubresourceFilter.PageLoad.RedirectChainMatchPattern) as a fallback for anything
that doesn't fit into your other cases. I don't consider this an intuitive use.
If you want to have a fallback for the non-specified cases, you should should
create a suffix for that purpose.

Let me know if I'm interpreting the code wrong though!

[melandory, 2017/03/09 15:48:45]

It's true for this CL that 2 types of histogram will be recorded: with suffix
and without.
But for the followup CL, the histogram without suffix should never be recorded.
But I can add a suffix for no suffix case which should not be recorded if you
prefer.

[jwd, 2017/03/10 16:43:25]

Ah ok, I didn't see that it wouldn't be recorded in the follow up cl, it's
clearer now thanks!  It's fine how it is then.

+</histogram_suffixes>
+
 <histogram_suffixes name="SyncModelType" separator=".">
   <suffix name="APP" label="APP"/>
   <suffix name="APP_LIST" label="APP_LIST"/>