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/autofill/core/browser

This CL was uploaded by git cl split.

R=parastoog@google.com

Bug: 1018667
Change-Id: I6b3dd0ae0eb6b701e10c1102227cabc7b027742d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1916823
Auto-Submit: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Parastoo Geranmayeh <parastoog@google.com>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715273}

components/autofill/core/browser/form_structure_unittest.cc

@@ -25,7 +25,6 @@
 #include "components/autofill/core/common/form_field_data.h"
 #include "components/autofill/core/common/password_form.h"
 #include "components/autofill/core/common/signatures_util.h"
-#include "components/variations/entropy_provider.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "url/gurl.h"
 
@@ -131,19 +130,22 @@ class FormStructureTest : public testing::Test {
     return form_structure.ShouldBeUploaded();
   }
 
-  void DisableAutofillMetadataFieldTrial() { field_trial_list_.reset(); }
+  void DisableAutofillMetadataFieldTrial() {
+    field_trial_ = nullptr;
+    scoped_feature_list_.Reset();
+    scoped_feature_list_.Init();
+  }
 
  private:
   void EnableAutofillMetadataFieldTrial() {
-    field_trial_list_.reset();
-    field_trial_list_.reset(new base::FieldTrialList(
-        std::make_unique<variations::SHA1EntropyProvider>("foo")));
+    scoped_feature_list_.Reset();
+    scoped_feature_list_.Init();
     field_trial_ = base::FieldTrialList::CreateFieldTrial(
         "AutofillFieldMetadata", "Enabled");
     field_trial_->group();
   }
 
-  std::unique_ptr<base::FieldTrialList> field_trial_list_;
+  base::test::ScopedFeatureList scoped_feature_list_;
   scoped_refptr<base::FieldTrial> field_trial_;
 };