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/metrics/perf

This CL was uploaded by git cl split.

R=gmx@chromium.org

Bug: 1018667
Change-Id: I8ad604b8c375a8fbe4f3ba49c1f51940fe2adbb6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1917504
Auto-Submit: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Gabriel Marin <gmx@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715760}

chrome/browser/metrics/perf/perf_events_collector_unittest.cc

@@ -722,7 +722,7 @@ TEST_F(PerfCollectorTest, JankinessCollectionDurationElapsed) {
 
 class PerfCollectorCollectionParamsTest : public testing::Test {
  public:
-  PerfCollectorCollectionParamsTest() : field_trial_list_(nullptr) {}
+  PerfCollectorCollectionParamsTest() {}
 
   void TearDown() override {
     variations::testing::ClearAllVariationParams();
@@ -731,8 +731,6 @@ class PerfCollectorCollectionParamsTest : public testing::Test {
  protected:
   content::BrowserTaskEnvironment task_environment_;
 
-  base::FieldTrialList field_trial_list_;
-
   DISALLOW_COPY_AND_ASSIGN(PerfCollectorCollectionParamsTest);
 };
 

chrome/browser/metrics/perf/profile_provider_unittest_main.cc

@@ -118,7 +118,7 @@ class TestProfileProvider : public ProfileProvider {
 // real collections from debugd.
 class ProfileProviderRealCollectionTest : public testing::Test {
  public:
-  ProfileProviderRealCollectionTest() : field_trial_list_(nullptr) {}
+  ProfileProviderRealCollectionTest() {}
 
   void SetUp() override {
     chromeos::DBusThreadManager::Initialize();
@@ -167,7 +167,6 @@ class ProfileProviderRealCollectionTest : public testing::Test {
   // last.
   content::BrowserTaskEnvironment task_environment_;
 
-  base::FieldTrialList field_trial_list_;
   scoped_refptr<base::FieldTrial> field_trial_;
 
   std::unique_ptr<TestProfileProvider> profile_provider_;