[HttpCache] Measure causes for cache validation requests

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 9087c187d2f21e8c7d6daa12ec102e82489f20dc..1cd97bd249ac5caab8803f505b3622c293d6ab2e 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -17533,6 +17533,11 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <summary>Net error results from restartable cache read errors.</summary>
 </histogram>
 
+<histogram name="HttpCache.ValidationCause" enum="HttpCacheValidationCause">
+  <owner>jkarlin@chromium.org</owner>
+  <summary>For each validation attempt, the cause for the validation.</summary>
+</histogram>
+
 <histogram name="HttpCache.Vary" enum="VaryType">
   <obsolete>
     Deprecated as of 11/2014.
@@ -74654,6 +74659,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="6" label="CantConditionalize"/>
 </enum>
 
+<enum name="HttpCacheValidationCause" type="int">
+  <int value="0" label="Undefined"/>
+  <int value="1" label="Vary Mismatch"/>
+  <int value="2" label="Validate Flag"/>
+  <int value="3" label="Stale Resource"/>
+  <int value="4" label="Zero Freshness"/>
+</enum>
+
 <enum name="HttpHeaderParserEvent" type="int">
   <int value="0" label="PARSER_INVOKED"/>
   <int value="1" label="HTTP_09_RESPONSE"/>
@@ -89517,6 +89530,7 @@ To add a new entry, add it with any value and run test to compute valid value.
              content-length &gt;= 100 bytes."/>
   <suffix name="Font" label="Showing cache patterns only for font resources."/>
   <affected-histogram name="HttpCache.Pattern"/>
+  <affected-histogram name="HttpCache.ValidationCause"/>
 </histogram_suffixes>
 
 <histogram_suffixes name="CacheInstance" separator="." ordering="prefix">