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:
/ios/chrome

This CL was uploaded by git cl split.

R=rohitrao@chromium.org

Bug: 1018667
Change-Id: I3ce32f0feaa2d652f5f4b8c68ae93e337e5e16be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1916824
Auto-Submit: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715343}

ios/chrome/browser/notification_promo_unittest.cc

@@ -13,7 +13,6 @@
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_util.h"
 #include "base/strings/utf_string_conversions.h"
-#include "base/test/mock_entropy_provider.h"
 #include "base/time/time.h"
 #include "base/values.h"
 #include "components/prefs/testing_pref_service.h"
@@ -51,8 +50,6 @@ class NotificationPromoTest : public PlatformTest {
  public:
   NotificationPromoTest()
       : notification_promo_(&local_state_),
-        field_trial_list_(std::make_unique<base::FieldTrialList>(
-            std::make_unique<base::MockEntropyProvider>())),
         received_notification_(false),
         start_(0.0),
         end_(0.0),
@@ -293,7 +290,6 @@ class NotificationPromoTest : public PlatformTest {
 
  private:
   NotificationPromo notification_promo_;
-  std::unique_ptr<base::FieldTrialList> field_trial_list_;
   bool received_notification_;
   base::Value test_json_;