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

net/base/host_resolver_impl.cc

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "net/base/host_resolver_impl.h"
 
 #if defined(OS_WIN)
 #include <Winsock2.h>
 #elif defined(OS_POSIX)
 #include <netdb.h>
@@ skipping 536 matching lines @@
         DNS_HISTOGRAM("DNS.ResolveSpeculativeFail", duration);
       }
       UMA_HISTOGRAM_CUSTOM_ENUMERATION(kOSErrorsForGetAddrinfoHistogramName,
                                        std::abs(os_error_),
                                        GetAllGetAddrinfoOSErrors());
     }
     DCHECK_LT(category, static_cast<int>(RESOLVE_MAX));  // Be sure it was set.
 
     UMA_HISTOGRAM_ENUMERATION("DNS.ResolveCategory", category, RESOLVE_MAX);
 
-    static bool show_speculative_experiment_histograms =
-        base::FieldTrialList::Find("DnsImpact") &&
-        !base::FieldTrialList::Find("DnsImpact")->group_name().empty();
+    static const bool show_speculative_experiment_histograms =
+        base::FieldTrialList::TrialExists("DnsImpact");
     if (show_speculative_experiment_histograms) {
       UMA_HISTOGRAM_ENUMERATION(
           base::FieldTrial::MakeName("DNS.ResolveCategory", "DnsImpact"),
           category, RESOLVE_MAX);
       if (RESOLVE_SUCCESS == category) {
         DNS_HISTOGRAM(base::FieldTrial::MakeName("DNS.ResolveSuccess",
                                                  "DnsImpact"), duration);
       }
     }
-    static bool show_parallelism_experiment_histograms =
-        base::FieldTrialList::Find("DnsParallelism") &&
-        !base::FieldTrialList::Find("DnsParallelism")->group_name().empty();
+    static const bool show_parallelism_experiment_histograms =
+        base::FieldTrialList::TrialExists("DnsParallelism");
     if (show_parallelism_experiment_histograms) {
       UMA_HISTOGRAM_ENUMERATION(
           base::FieldTrial::MakeName("DNS.ResolveCategory", "DnsParallelism"),
           category, RESOLVE_MAX);
       if (RESOLVE_SUCCESS == category) {
         DNS_HISTOGRAM(base::FieldTrial::MakeName("DNS.ResolveSuccess",
                                                  "DnsParallelism"), duration);
       }
     }
   }
@@ skipping 842 matching lines @@
     additional_resolver_flags_ |= HOST_RESOLVER_LOOPBACK_ONLY;
   } else {
     additional_resolver_flags_ &= ~HOST_RESOLVER_LOOPBACK_ONLY;
   }
 #endif
   AbortAllInProgressJobs();
   // |this| may be deleted inside AbortAllInProgressJobs().
 }
 
 }  // namespace net