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 a4cb8afc145c8575e7db12d87e15abcc913e3096..01b6d103ea861d152b0f9183ad90826d1702d88e 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -68716,9 +68716,15 @@ 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>
+  <summary>
+    Total number of server redirects during the navigation. This histogram is
+    recorded in several flavours: with prefix SubresourceFilterOnly in case the
+    redirect chain contains url from the Safe Browsing SubresourceFilter list
+    and without the prefix for all other redirect chains.

[engedy, 2017/03/21 13:44:16]

nit: Is this last line still up-to-date? I don't see any code paths recording
values without a suffix. (Which seems the expected behavior.)

[melandory, 2017/03/24 15:49:28]

Done.

+  </summary>
   <suffix name="SocialEngineeringAdsInterstitial"/>
   <suffix name="PhishingInterstitial"/>
-  <summary>Total length of the server redirects during the navigation.</summary>
+  <suffix name="SubresourceFilterOnly"/>
 </histogram>
 
 <histogram name="SubresourceFilter.PageLoad.RedirectChainMatchPattern"
@@ -68726,13 +68732,17 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <owner>melandory@chromium.org</owner>
   <suffix name="SocialEngineeringAdsInterstitial"/>
   <suffix name="PhishingInterstitial"/>
+  <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.
     For example, for the redirect chain A-B-C-D metric tracks: 1. If initial URL
     (A) was on a Safe Browsing blacklist. 2. If any if middle urls (B, C) were
     on a Safe Browsing blacklist. 3. If committed URL (B) was on a Safe Browsing
-    blacklist.
+    blacklist. This histogram is recorded in several flavours: with prefix
+    SubresourceFilterOnly in case the redirect chain contains url from the Safe
+    Browsing SubresourceFilter list and without the prefix for all other
+    redirect chains.
   </summary>
 </histogram>
 
@@ -122132,6 +122142,7 @@ from previous Chrome versions.
 <histogram_suffixes name="SubresourceFilterOnlyExperiment" separator=".">
   <suffix name="SocialEngineeringAdsInterstitial"/>
   <suffix name="PhishingInterstitial"/>
+  <suffix name="SubresourceFilterOnly"/>
   <affected-histogram name="SubresourceFilter.PageLoad.RedirectChainLength"/>
   <affected-histogram
       name="SubresourceFilter.PageLoad.RedirectChainMatchPattern"/>