Record SB subresource metrics separately from non-subresource metrics

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 e2877bc1b65796897ebbbc3b58ea4a5181d63683..bb159aadbbf95fcaef265c7104263cef16ac2ffe 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -13520,7 +13520,10 @@ Therefore, the affected-histogram name has to have at least one dot in it.
 
 <histogram name="interstitial.decision" enum="SecurityInterstitialDecision">
   <owner>felt@chromium.org</owner>
-  <summary>User decision when presented with a security interstitial.</summary>
+  <summary>
+    User decision when presented with a security interstitial. As of M44,
+    subresource interstitials were split into their own entries.
+  </summary>
 </histogram>
 
 <histogram name="interstitial.decision.repeat_visit"
@@ -13528,14 +13531,18 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <owner>felt@chromium.org</owner>
   <summary>
     User decision when presented with a security interstitial, on a site that
-    the user had previously visited.
+    the user had previously visited.  As of M44, subresource interstitials were
+    split into their own entries.
   </summary>
 </histogram>
 
 <histogram name="interstitial.interaction"
     enum="SecurityInterstitialInteraction">
   <owner>felt@chromium.org</owner>
-  <summary>User interactions with a security interstitial.</summary>
+  <summary>
+    User interactions with a security interstitial.  As of M44, subresource
+    interstitials were split into their own entries.
+  </summary>
 </histogram>
 
 <histogram name="interstitial.ssl" enum="SSLResponseTypesV2">
@@ -69605,8 +69612,11 @@ To add a new entry, add it with any value and run test to compute valid value.
     ordering="prefix">
   <suffix name="bad_clock"/>
   <suffix name="harmful"/>
+  <suffix name="harmful_subresource"/>
   <suffix name="malware"/>
+  <suffix name="malware_subresource"/>
   <suffix name="phishing"/>
+  <suffix name="phishing_subresource"/>
   <suffix name="ssl_nonoverridable"/>
   <suffix name="ssl_overridable"/>
   <affected-histogram name="interstitial.decision"/>