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 fb7ebd76f660639411317639e60ea3ab5f6dbf6b..548d6b3cf50e7e5caa2b04cedee3d0411183ba30 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -68622,7 +68622,12 @@ 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 length of the server redirects during the navigation.</summary>
+  <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.
+  </summary>
 </histogram>
 
 <histogram name="SubresourceFilter.PageLoad.RedirectChainMatchPattern"
@@ -68634,7 +68639,10 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
     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>
 
@@ -121996,6 +122004,13 @@ from previous Chrome versions.
   <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"/>
+</histogram_suffixes>
+
 <histogram_suffixes name="SyncModelType" separator=".">
   <suffix name="APP" label="APP"/>
   <suffix name="APP_LIST" label="APP_LIST"/>