[HttpCache] Measure causes for cache validation requests

net/http/http_cache_transaction.h

Index: net/http/http_cache_transaction.h
diff --git a/net/http/http_cache_transaction.h b/net/http/http_cache_transaction.h
index 0c9b11253543ac44c65986bdd743c45870c03972..8e26337820396e3fd9f1825012df736f2c159dcd 100644
--- a/net/http/http_cache_transaction.h
+++ b/net/http/http_cache_transaction.h
@@ -246,6 +246,18 @@ class HttpCache::Transaction : public HttpTransaction {
     PATTERN_MAX,
   };
 
+  // Used for categorizing validation triggers in histograms.
+  // NOTE: This enumeration is used in histograms, so please do not add entries
+  // in the middle.
+  enum ValidationCause {

[gavinp, 2016/05/12 15:02:02]

Could we use an enum class here?

[jkarlin, 2016/05/13 15:06:51]

We could, but we'd still have to static_cast the histogram calls. 

+    VALIDATION_CAUSE_UNDEFINED,
+    VALIDATION_CAUSE_VARY_MISMATCH,
+    VALIDATION_CAUSE_VALIDATE_FLAG,
+    VALIDATION_CAUSE_STALE,
+    VALIDATION_CAUSE_ZERO_FRESHNESS,
+    VALIDATION_CAUSE_MAX
+  };
+
   // Runs the state transition loop. Resets and calls |callback_| on exit,
   // unless the return value is ERR_IO_PENDING.
   int DoLoop(int result);
@@ -468,6 +480,7 @@ class HttpCache::Transaction : public HttpTransaction {
 
   // Members used to track data for histograms.
   TransactionPattern transaction_pattern_;
+  ValidationCause validation_cause_;
   base::TimeTicks entry_lock_waiting_since_;
   base::TimeTicks first_cache_access_since_;
   base::TimeTicks send_request_since_;