Make ScopedFeatureList Field Trial Name Unique.

Make the name for the field trial created by ScopedFeatureList unique
so that different ScopedFeatureLists do not collide when the field
trial is stored globally.

Bug: none
Change-Id: Ibe0ebc6747868fd468e0c9455f21e990635aa1c4
Reviewed-on: https://chromium-review.googlesource.com/813102
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Gregory Chatzinoff <gchatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523360}

base/test/scoped_feature_list.cc

@@ -11,6 +11,7 @@
 #include "base/memory/ptr_util.h"
 #include "base/metrics/field_trial_param_associator.h"
 #include "base/stl_util.h"
+#include "base/strings/string_number_conversions.h"
 #include "base/strings/string_split.h"
 #include "base/strings/string_util.h"
 
@@ -191,8 +192,17 @@ void ScopedFeatureList::InitAndEnableFeatureWithParameters(
     field_trial_list_ = std::make_unique<base::FieldTrialList>(nullptr);
   }
 
-  std::string kTrialName = "scoped_feature_list_trial_name";
+  // TODO(crbug.com/794021) Remove this unique field trial name hack when there
+  // is a cleaner solution.
+  // Ensure that each call to this method uses a distinct field trial name.
+  // Otherwise, nested calls might fail due to the shared FieldTrialList
+  // already having the field trial registered.
+  static int num_calls = 0;
+  ++num_calls;
+  std::string kTrialName =
+      "scoped_feature_list_trial_name" + base::NumberToString(num_calls);
   std::string kTrialGroup = "scoped_feature_list_trial_group";
+
   field_trial_override_ =
       base::FieldTrialList::CreateFieldTrial(kTrialName, kTrialGroup);
   DCHECK(field_trial_override_);

base/test/scoped_feature_list_unittest.cc

@@ -166,7 +166,7 @@ TEST_F(ScopedFeatureListTest, OverrideWithFeatureParameters) {
     EXPECT_TRUE(FeatureList::IsEnabled(kTestFeature1));
     EXPECT_TRUE(FeatureList::IsEnabled(kTestFeature2));
     EXPECT_EQ("", GetFieldTrialParamValueByFeature(kTestFeature1, kParam));
-    EXPECT_EQ("", GetFieldTrialParamValueByFeature(kTestFeature2, kParam));
+    EXPECT_EQ(kValue, GetFieldTrialParamValueByFeature(kTestFeature2, kParam));
     EXPECT_EQ(trial.get(), FeatureList::GetFieldTrial(kTestFeature1));
     EXPECT_NE(nullptr, FeatureList::GetFieldTrial(kTestFeature2));
   }