Log histograms for preloads and preload misses

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 7734ffeaccfaf8639f65c1d822fb7ceab9899318..4d85c713a355bdfed3239361890c9bc5f1385df2 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -40607,6 +40607,18 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <summary>The amount of time that elapsed during CreateProfilePrefs.</summary>
 </histogram>
 
+<histogram name="PreloadScanner.Counts" units="preloads">
+  <owner>csharrison@chromium.org</owner>
+  <summary>The number of preloads generated by the preload scanner.</summary>
+</histogram>
+
+<histogram name="PreloadScanner.Counts.Miss" units="preloads">
+  <owner>csharrison@chromium.org</owner>
+  <summary>
+    The number of unused preloads generated by the preload scanner.
+  </summary>
+</histogram>
+
 <histogram name="PreloadScanner.DocumentWrite.ExecutionTime.Failure" units="ms">
   <owner>csharrison@chromium.org</owner>
   <summary>
@@ -94521,6 +94533,8 @@ To add a new entry, add it with any value and run test to compute valid value.
   <affected-histogram name="Blink.MemoryCache.RevalidationPolicy"/>
   <affected-histogram name="Blink.MemoryCache.RevalidationPolicy.Dead"/>
   <affected-histogram name="Blink.MemoryCache.RevalidationPolicy.Preload"/>
+  <affected-histogram name="PreloadScanner.Counts"/>
+  <affected-histogram name="PreloadScanner.Counts.Miss"/>
 </histogram_suffixes>
 
 <histogram_suffixes name="SafeBrowsingLists" separator=".">