Add one-time randomization support for FieldTrial, and the ability to

net/http/http_network_transaction.cc

Index: net/http/http_network_transaction.cc
diff --git a/net/http/http_network_transaction.cc b/net/http/http_network_transaction.cc
index 46858e627f09bc07ce36639d7b70684d6c03baf5..604d793f96f096d730208bd376a75c3d9e4cc5e3 100644
--- a/net/http/http_network_transaction.cc
+++ b/net/http/http_network_transaction.cc
@@ -1007,9 +1007,8 @@ void HttpNetworkTransaction::LogTransactionConnectedMetrics() {
         base::TimeDelta::FromMilliseconds(1), base::TimeDelta::FromMinutes(10),
         100);
 
-  static bool use_conn_impact_histogram(
-      base::FieldTrialList::Find("ConnCountImpact") &&
-      !base::FieldTrialList::Find("ConnCountImpact")->group_name().empty());
+  static const bool use_conn_impact_histogram =
+      base::FieldTrialList::TrialExists("ConnCountImpact");
   if (use_conn_impact_histogram) {
     UMA_HISTOGRAM_CLIPPED_TIMES(
         base::FieldTrial::MakeName("Net.Transaction_Connected_New",
@@ -1020,8 +1019,8 @@ void HttpNetworkTransaction::LogTransactionConnectedMetrics() {
     }
   }
 
-  static bool use_spdy_histogram(base::FieldTrialList::Find("SpdyImpact") &&
-      !base::FieldTrialList::Find("SpdyImpact")->group_name().empty());
+  static const bool use_spdy_histogram =
+      base::FieldTrialList::TrialExists("SpdyImpact");
   if (use_spdy_histogram && response_.was_npn_negotiated) {
     UMA_HISTOGRAM_CLIPPED_TIMES(
       base::FieldTrial::MakeName("Net.Transaction_Connected_Under_10",