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:
/chrome/browser/ui/app_list

This CL was uploaded by git cl split.

R=khmel@chromium.org

Bug: 1018667
Change-Id: Ie15d91cf04bff703c332ad2ee74d81cda8ec7457
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1916472
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Auto-Submit: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Yury Khmel <khmel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715376}

chrome/browser/ui/app_list/search/answer_card/answer_card_search_provider_unittest.cc

@@ -55,7 +55,7 @@ std::unique_ptr<KeyedService> CreateTemplateURLService(
 
 class AnswerCardSearchProviderTest : public AppListTestBase {
  public:
-  AnswerCardSearchProviderTest() : field_trial_list_(nullptr) {}
+  AnswerCardSearchProviderTest() = default;
 
   FakeAppListModelUpdater* GetModelUpdater() const {
     return model_updater_.get();
@@ -98,7 +98,6 @@ class AnswerCardSearchProviderTest : public AppListTestBase {
   std::unique_ptr<FakeAppListModelUpdater> model_updater_;
   std::unique_ptr<AnswerCardSearchProvider> provider_;
   std::unique_ptr<::test::TestAppListControllerDelegate> controller_;
-  base::FieldTrialList field_trial_list_;
   base::test::ScopedFeatureList scoped_feature_list_;
 
   DISALLOW_COPY_AND_ASSIGN(AnswerCardSearchProviderTest);