Add metrics for tracking subresource filter activation suppression.

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 f7fd8937c039f19f0327adf824671dab93cf2849..f8da3d7b65e4c025e6fa19a894e345a788b250fe 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -43938,6 +43938,19 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <summary>Events related to Google CAPTCHA pages being seen by users.</summary>
 </histogram>
 
+<histogram name="PageLoad.Clients.SubresourceFilter.ActivationDecision"
+    enum="SubresourceFilterActivationDecision">
+  <owner>bmcquade@chromium.org</owner>
+  <summary>
+    Records the policy decision to activate subresource filtering for a page
+    load. 'Activated' indicates that subresource filtering was activated. All
+    other reasons indicate that subresource filtering was not activated. Page
+    loads where subresource filtering was activated ('Activated') and at least
+    one subresource matched the subresource filter are counted in
+    PageLoad.Clients.SubresourceFilter.Count.
+  </summary>
+</histogram>
+
 <histogram name="PageLoad.Clients.SubresourceFilter.Count" enum="Boolean">
   <owner>bmcquade@chromium.org</owner>
   <summary>
@@ -108588,6 +108601,15 @@ value.
   <int value="2" label="MemoryCached StyleSheetContents was reused"/>
 </enum>
 
+<enum name="SubresourceFilterActivationDecision" type="int">
+  <int value="0" label="Unknown"/>
+  <int value="1" label="Disabled"/>
+  <int value="2" label="Unsupported scheme"/>
+  <int value="3" label="URL whitelisted"/>
+  <int value="4" label="Activation list not matched"/>
+  <int value="5" label="Activated"/>

[Ilya Sherman, 2017/02/21 22:58:24]

Optional: You might want to arrange the enum so that this is item 1 (following
"Unknown").  That way, if you ever add reasons for why activation failed, this
success state doesn't end up buried in the middle of the list.

[Bryan McQuade, 2017/02/22 01:03:47]

Yes, great idea. Done, thanks!

+</enum>
+
 <enum name="SubresourceFilterActivationState" type="int">
   <int value="0" label="Disabled"/>
   <int value="1" label="DryRun"/>
@@ -117717,6 +117739,8 @@ value.
       label="Restricted to new navigations (link clicks, URLs typed into the
              URL box, etc)."/>
   <affected-histogram
+      name="PageLoad.Clients.SubresourceFilter.ActivationDecision"/>
+  <affected-histogram
       name="PageLoad.PaintTiming.NavigationToFirstContentfulPaint"/>
   <affected-histogram name="PageLoad.ParseTiming.NavigationToParseStart"/>
 </histogram_suffixes>