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/search

This CL was uploaded by git cl split.

R=kmilka@chromium.org

Bug: 1018667
Change-Id: I6006941d3c12b2a041306d5dfd0e25b20348ef52
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1917507
Auto-Submit: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Kyle Milka <kmilka@chromium.org>
Commit-Queue: Kyle Milka <kmilka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715712}

chrome/browser/search/instant_unittest_base.cc

@@ -22,11 +22,8 @@
 #include "components/search/search.h"
 #include "components/search_engines/template_url.h"
 #include "components/search_engines/template_url_service.h"
-#include "components/variations/entropy_provider.h"
 
 InstantUnitTestBase::InstantUnitTestBase() {
-  field_trial_list_.reset(new base::FieldTrialList(
-      std::make_unique<variations::SHA1EntropyProvider>("42")));
 }
 
 InstantUnitTestBase::~InstantUnitTestBase() {

chrome/browser/search/instant_unittest_base.h

@@ -7,7 +7,6 @@
 
 #include <string>
 
-#include "base/metrics/field_trial.h"
 #include "base/test/simple_test_clock.h"
 #include "build/build_config.h"
 #include "chrome/browser/search/instant_service.h"
@@ -34,7 +33,6 @@ class InstantUnitTestBase : public BrowserWithTestWindowTest {
 
   InstantService* instant_service_;
   TemplateURLService* template_url_service_;
-  std::unique_ptr<base::FieldTrialList> field_trial_list_;
   base::SimpleTestClock* clock_;
 
  private: