Do not instantiate base::FieldTrialList explicitly in tests

The test suite already instantiates a FieldTrialList as per
https://chromium-review.googlesource.com/c/chromium/src/+/1883567 so
it's no longer necessary to do so explicitly in tests.

This patch addresses unit tests under:
/components/previews

This CL was uploaded by git cl split.

R=dougarnett@chromium.org

Bug: 1018667
Change-Id: I961f06327e484ce15d377f60fb7fd1b1d2f12050
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1917364
Auto-Submit: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Doug Arnett <dougarnett@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715745}

components/previews/content/previews_decider_impl_unittest.cc

@@ -433,8 +433,7 @@ class PreviewsDeciderImplTest
     : public optimization_guide::ProtoDatabaseProviderTestBase {
  public:
   PreviewsDeciderImplTest()
-      : field_trial_list_(nullptr),
-        previews_decider_impl_(nullptr),
+      : previews_decider_impl_(nullptr),
         optimization_guide_service_(
             task_environment_.GetMainThreadTaskRunner()) {
     clock_.SetNow(base::Time::Now());
@@ -514,7 +513,6 @@ class PreviewsDeciderImplTest
 
  private:
   base::test::TaskEnvironment task_environment_;
-  base::FieldTrialList field_trial_list_;
   TestPreviewsDeciderImpl* previews_decider_impl_;
   optimization_guide::OptimizationGuideService optimization_guide_service_;
   TestPreviewsOptimizationGuide* previews_opt_guide_;

components/previews/core/previews_black_list_unittest.cc

@@ -89,7 +89,7 @@ class TestPreviewsBlackList : public PreviewsBlackList {
 
 class PreviewsBlackListTest : public testing::Test {
  public:
-  PreviewsBlackListTest() : field_trial_list_(nullptr), passed_reasons_({}) {}
+  PreviewsBlackListTest() : passed_reasons_({}) {}
   ~PreviewsBlackListTest() override {}
 
   void TearDown() override { variations::testing::ClearAllVariationParams(); }
@@ -159,7 +159,6 @@ class PreviewsBlackListTest : public testing::Test {
 
   base::SimpleTestClock test_clock_;
   std::map<std::string, std::string> params_;
-  base::FieldTrialList field_trial_list_;
 
   std::unique_ptr<TestPreviewsBlackList> black_list_;
   std::vector<PreviewsEligibilityReason> passed_reasons_;

components/previews/core/previews_experiments_unittest.cc

@@ -53,8 +53,6 @@ TEST(PreviewsExperimentsTest, TestParamsForBlackListAndOffline) {
             params::GetECTThresholdForPreview(PreviewsType::NOSCRIPT));
   EXPECT_EQ(0, params::OfflinePreviewsVersion());
 
-  base::FieldTrialList field_trial_list(nullptr);
-
   // Set some custom params. Somewhat random yet valid values.
   std::map<std::string, std::string> custom_params = {
       {"per_host_max_stored_history_length", "3"},
@@ -245,7 +243,6 @@ TEST(PreviewsExperimentsTest, TestShouldExcludeMediaSuffix) {
 }
 
 TEST(PreviewsExperimentsTest, TestPreviewsEligibleForUserConsistentStudy) {
-  base::FieldTrialList field_trial_list(nullptr);
   EXPECT_TRUE(base::FieldTrialList::CreateFieldTrial("CoinFlipPreviews",
                                                      "NonCoinFlipGroup"));
 
@@ -347,10 +344,6 @@ TEST(PreviewsExperimentsTest, TestPreviewsEligibleForUserConsistentStudy) {
 }
 
 TEST(PreviewsExperimentsTest, TestLitePageServerPreviewsCoinFlipExperiment) {
-  base::FieldTrialList field_trial_list(nullptr);
-  EXPECT_TRUE(base::FieldTrialList::CreateFieldTrial(
-      kPreviewsFieldTrial, "LitePageServerPreviewsCoinFlipExperimentGroup"));
-
   base::test::ScopedFeatureList scoped_feature_list;
   scoped_feature_list.InitWithFeaturesAndParameters(
       {{features::kLitePageServerPreviewsUserConsistentStudy,
@@ -363,6 +356,9 @@ TEST(PreviewsExperimentsTest, TestLitePageServerPreviewsCoinFlipExperiment) {
       {features::kEligibleForUserConsistentStudy,
        features::kDeferAllScriptPreviews});
 
+  EXPECT_TRUE(base::FieldTrialList::CreateFieldTrial(
+      kPreviewsFieldTrial, "LitePageServerPreviewsCoinFlipExperimentGroup"));
+
   EXPECT_TRUE(params::IsLitePageServerPreviewsEnabled());
   EXPECT_EQ(222, params::LitePageServerPreviewsVersion());
   EXPECT_EQ(GURL("https://coinflippreviewhost.org/"),
@@ -371,10 +367,6 @@ TEST(PreviewsExperimentsTest, TestLitePageServerPreviewsCoinFlipExperiment) {
 }
 
 TEST(PreviewsExperimentsTest, TestDeferAllScriptPreviewsCoinFlipExperiment) {
-  base::FieldTrialList field_trial_list(nullptr);
-  EXPECT_TRUE(base::FieldTrialList::CreateFieldTrial(
-      kPreviewsFieldTrial, "DeferAllScriptPreviewsCoinFlipExperimentGroup"));
-
   base::test::ScopedFeatureList scoped_feature_list;
   scoped_feature_list.InitWithFeaturesAndParameters(
       {{features::kDeferAllScriptPreviewsUserConsistentStudy,
@@ -383,6 +375,9 @@ TEST(PreviewsExperimentsTest, TestDeferAllScriptPreviewsCoinFlipExperiment) {
       {features::kEligibleForUserConsistentStudy,
        features::kLitePageServerPreviews});
 
+  EXPECT_TRUE(base::FieldTrialList::CreateFieldTrial(
+      kPreviewsFieldTrial, "DeferAllScriptPreviewsCoinFlipExperimentGroup"));
+
   EXPECT_TRUE(params::IsDeferAllScriptPreviewsEnabled());
   EXPECT_EQ(444, params::DeferAllScriptPreviewsVersion());