Feature to disable field trials in old versions of Chromium. Field trials...

chrome/browser/io_thread.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 "chrome/browser/io_thread.h"
 
 #include <vector>
 
 #include "base/command_line.h"
 #include "base/debug/leak_tracker.h"
@@ skipping 52 matching lines @@
     } else {
       LOG(ERROR) << "Invalid switch for host resolver parallelism: " << s;
     }
   } else {
     // Set up a field trial to see what impact the total number of concurrent
     // resolutions have on DNS resolutions.
     base::FieldTrial::Probability kDivisor = 1000;
     // For each option (i.e., non-default), we have a fixed probability.
     base::FieldTrial::Probability kProbabilityPerGroup = 100;  // 10%.
 
+    // After June 30, 2011 builds, it will always be in default group
+    // (parallel_default).
     scoped_refptr<base::FieldTrial> trial(
-        new base::FieldTrial("DnsParallelism", kDivisor));
+        new base::FieldTrial(
+            "DnsParallelism", kDivisor, "parallel_default", 2011, 6, 30));
 
     // List options with different counts.
     // Firefox limits total to 8 in parallel, and default is currently 50.
     int parallel_6 = trial->AppendGroup("parallel_6", kProbabilityPerGroup);
     int parallel_7 = trial->AppendGroup("parallel_7", kProbabilityPerGroup);
     int parallel_8 = trial->AppendGroup("parallel_8", kProbabilityPerGroup);
     int parallel_9 = trial->AppendGroup("parallel_9", kProbabilityPerGroup);
     int parallel_10 = trial->AppendGroup("parallel_10", kProbabilityPerGroup);
     int parallel_14 = trial->AppendGroup("parallel_14", kProbabilityPerGroup);
     int parallel_20 = trial->AppendGroup("parallel_20", kProbabilityPerGroup);
 
-    trial->AppendGroup("parallel_default",
-                        base::FieldTrial::kAllRemainingProbability);
-
     if (trial->group() == parallel_6)
       parallelism = 6;
     else if (trial->group() == parallel_7)
       parallelism = 7;
     else if (trial->group() == parallel_8)
       parallelism = 8;
     else if (trial->group() == parallel_9)
       parallelism = 9;
     else if (trial->group() == parallel_10)
       parallelism = 10;
@@ skipping 407 matching lines @@
     net::HostCache* host_cache =
         globals_->host_resolver.get()->GetAsHostResolverImpl()->cache();
     if (host_cache)
       host_cache->clear();
   }
   // Clear all of the passively logged data.
   // TODO(eroman): this is a bit heavy handed, really all we need to do is
   //               clear the data pertaining to off the record context.
   net_log_->ClearAllPassivelyCapturedEvents();
 }