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/profile_resetter

This CL was uploaded by git cl split.

R=battre@chromium.org

Bug: 1018667
Change-Id: Ib05a1b4e6df9e41126a2810d800be92907ee8639
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1917228
Auto-Submit: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Dominic Battré <battre@chromium.org>
Commit-Queue: Dominic Battré <battre@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715605}

chrome/browser/profile_resetter/triggered_profile_resetter_win_unittest.cc

@@ -17,7 +17,6 @@
 #include "chrome/common/pref_names.h"
 #include "chrome/test/base/testing_profile.h"
 #include "components/prefs/pref_service.h"
-#include "components/variations/entropy_provider.h"
 #include "content/public/test/browser_task_environment.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
@@ -34,8 +33,6 @@ class TriggeredProfileResetterTest : public testing::Test {
         override_manager_.OverrideRegistry(HKEY_CURRENT_USER));
 
     // Activate the triggered reset field trial for these tests.
-    field_trial_list_.reset(new base::FieldTrialList(
-        std::make_unique<variations::SHA1EntropyProvider>("foo")));
     base::FieldTrial* trial = base::FieldTrialList::CreateFieldTrial(
         "TriggeredResetFieldTrial", "On");
     trial->group();
@@ -61,7 +58,6 @@ class TriggeredProfileResetterTest : public testing::Test {
 
  private:
   registry_util::RegistryOverrideManager override_manager_;
-  std::unique_ptr<base::FieldTrialList> field_trial_list_;
 };
 
 TEST_F(TriggeredProfileResetterTest, HasResetTriggerAndClear) {