Update a comment in network time test helper

components/network_time/network_time_test_utils.cc

 // Copyright 2016 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 "components/network_time/network_time_test_utils.h"
 
 #include "base/feature_list.h"
 #include "base/memory/ptr_util.h"
 #include "base/metrics/field_trial.h"
 #include "base/strings/string_number_conversions.h"
@@ skipping 77 matching lines @@
 
   // There are 3 things here: a FieldTrial, a FieldTrialList, and a
   // FeatureList.  Don't get confused!  The FieldTrial is reference-counted,
   // and a reference is held by the FieldTrialList.  The FieldTrialList and
   // FeatureList are both singletons.  The authorized way to reset the former
   // for testing is to destruct it (above).  The latter, by contrast, should
   // should already start in a clean state and can be manipulated via the
   // ScopedFeatureList helper class. If this comment was useful to you
   // please send me a postcard.
 
-  field_trial_list_.reset();  // Averts a CHECK fail in constructor below.
+  // SetNetworkQueriesWithVariationsService() is usually called during test
+  // fixture setup (to establish a default state) and then again in certain
+  // tests that want to set special params. FieldTrialList is meant to be a
+  // singleton with only one instance existing at once, and the constructor
+  // fails a CHECK if this is violated. To allow these duplicate calls to this
+  // method, any existing FieldTrialList must be destroyed before creating a new
+  // one.

[meacer, 2017/06/07 17:29:20]

nit: Would it make sense to mention the alternative as well? (remove
SetNetworkQueriesWithVariationsService from here and call it separately in each
test)

+  field_trial_list_.reset();
   field_trial_list_.reset(
       new base::FieldTrialList(base::MakeUnique<base::MockEntropyProvider>()));
 
   // refcounted, and reference held by the singleton FieldTrialList.
   base::FieldTrial* trial = base::FieldTrialList::FactoryGetFieldTrial(
       kTrialName, 100, kGroupName, 1971, 1, 1,
       base::FieldTrial::SESSION_RANDOMIZED, nullptr /* default_group_number */);
   ASSERT_TRUE(
       variations::AssociateVariationParams(kTrialName, kGroupName, params));
 
   std::unique_ptr<base::FeatureList> feature_list(new base::FeatureList);
   feature_list->RegisterFieldTrialOverride(
       kFeature.name, enable ? base::FeatureList::OVERRIDE_ENABLE_FEATURE
                             : base::FeatureList::OVERRIDE_DISABLE_FEATURE,
       trial);
   scoped_feature_list_.reset(new base::test::ScopedFeatureList);
   scoped_feature_list_->InitWithFeatureList(std::move(feature_list));
 }
 
 }  // namespace network_time