Add subresource filter page load metrics observer

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 0cd5aa6f3f6302d7dd861e3fe4ecfdff2cdc69b5..a933762475086aad6acbf444d97c25e31d39e926 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -43738,6 +43738,17 @@ 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.Count" enum="Boolean">
+  <owner>bmcquade@chromium.org</owner>
+  <summary>
+    Records 'true' for page loads where the subresource filter matched a
+    subresource loaded by the page. Includes dryrun matches. To compute the
+    percentage of total page loads affected by subresource filtering, divide by
+    the sum of counts for (PageLoad.ParseTiming.NavigationToParseStart +
+    PageLoad.ParseTiming.NavigationToParseStart.Background).
+  </summary>
+</histogram>
+
 <histogram name="PageLoad.CSSTiming.Parse.BeforeFirstContentfulPaint"
     units="ms">
   <owner>csharrison@chromium.org</owner>
@@ -117312,6 +117323,30 @@ value.
       name="PageLoad.Clients.ServiceWorker.PaintTiming.ParseStartToFirstContentfulPaint"/>
 </histogram_suffixes>
 
+<histogram_suffixes name="PageLoadMetricsClientsSubresourceFilter"
+    separator="." ordering="prefix">
+  <suffix name="Clients.SubresourceFilter"
+      label="For pages with filtered subresources. Includes dryrun matches."/>
+  <affected-histogram
+      name="PageLoad.DocumentTiming.NavigationToDOMContentLoadedEventFired"/>
+  <affected-histogram
+      name="PageLoad.DocumentTiming.NavigationToLoadEventFired"/>
+  <affected-histogram name="PageLoad.Experimental.Bytes.Cache"/>
+  <affected-histogram name="PageLoad.Experimental.Bytes.Network"/>
+  <affected-histogram name="PageLoad.Experimental.Bytes.Total"/>
+  <affected-histogram name="PageLoad.Experimental.CompletedResources.Cache"/>
+  <affected-histogram name="PageLoad.Experimental.CompletedResources.Network"/>
+  <affected-histogram name="PageLoad.Experimental.CompletedResources.Total"/>
+  <affected-histogram
+      name="PageLoad.Experimental.PaintTiming.NavigationToFirstMeaningfulPaint"/>
+  <affected-histogram
+      name="PageLoad.Experimental.PaintTiming.ParseStartToFirstMeaningfulPaint"/>
+  <affected-histogram
+      name="PageLoad.PaintTiming.NavigationToFirstContentfulPaint"/>
+  <affected-histogram
+      name="PageLoad.PaintTiming.ParseStartToFirstContentfulPaint"/>
+</histogram_suffixes>
+
 <histogram_suffixes name="PageLoadMetricsLoadType" separator=".">
   <suffix name="LoadType.Reload" label="Restricted to reloaded pages."/>
   <suffix name="LoadType.ForwardBackNavigation"