Add HttpCache.ExternallyConditionalized histogram.

net/http/http_cache_transaction.cc

Index: net/http/http_cache_transaction.cc
diff --git a/net/http/http_cache_transaction.cc b/net/http/http_cache_transaction.cc
index edc638905881acc3c46af34049c109050a37ea70..46d9b848cbf2c485ce189c5348071b35e694c150 100644
--- a/net/http/http_cache_transaction.cc
+++ b/net/http/http_cache_transaction.cc
@@ -238,6 +238,22 @@ void RecordNoStoreHeaderHistogram(int load_flags,
   }
 }
 
+// TODO(ricea): Add CACHE_USABLE_STALE once stale-while-revalidate CL landed.
+// TODO(ricea): Either remove this histogram or make it permanent by M40.
+void RecordExternallyConditionalizedHistogram(bool requires_validation) {
+  enum ExternallyConditionalizedType {
+    EXTERNALLY_CONDITIONALIZED_CACHE_REQUIRES_VALIDATION,
+    EXTERNALLY_CONDITIONALIZED_CACHE_USABLE,
+    EXTERNALLY_CONDITIONALIZED_MAX
+  };
+  ExternallyConditionalizedType type =
+      requires_validation ? EXTERNALLY_CONDITIONALIZED_CACHE_REQUIRES_VALIDATION
+                          : EXTERNALLY_CONDITIONALIZED_CACHE_USABLE;
+  UMA_HISTOGRAM_ENUMERATION("HttpCache.ExternallyConditionalized",
+                            type,
+                            EXTERNALLY_CONDITIONALIZED_MAX);
+}
+
 }  // namespace
 
 namespace net {
@@ -2226,6 +2242,11 @@ int HttpCache::Transaction::BeginExternallyConditionalizedRequest() {
     }
   }
 
+  // TODO(ricea): This calculation is expensive to perform just to collect
+  // statistics. Either remove it or use the result, depending on the result of
+  // the experiment.
+  RecordExternallyConditionalizedHistogram(RequiresValidation());
+
   next_state_ = STATE_SEND_REQUEST;
   return OK;
 }