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:
/components/sync

This CL was uploaded by git cl split.

R=vitaliii@chromium.org

Bug: 1018667
Change-Id: I361805aad758e19356c63f6321b890b9cd4715ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1917764
Auto-Submit: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: vitaliii <vitaliii@chromium.org>
Commit-Queue: vitaliii <vitaliii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715798}

components/sync/engine_impl/sync_manager_impl_unittest.cc

@@ -13,7 +13,6 @@
 #include "base/files/scoped_temp_dir.h"
 #include "base/format_macros.h"
 #include "base/location.h"
-#include "base/metrics/field_trial.h"
 #include "base/run_loop.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/stringprintf.h"
@@ -1981,8 +1980,6 @@ TEST_F(SyncManagerTest, UpdatePasswordReencryptEverything) {
 // written when it's applicable, namely that password specifics entity is marked
 // unsynced, when data was written to the unencrypted metadata field.
 TEST_F(SyncManagerTest, UpdatePasswordReencryptEverythingFillMetadata) {
-  base::FieldTrialList field_trial_list(nullptr);
-
   EXPECT_TRUE(SetUpEncryption(WRITE_TO_NIGORI, DEFAULT_ENCRYPTION));
   sync_pb::EntitySpecifics entity_specifics;
   {
@@ -2027,8 +2024,6 @@ TEST_F(SyncManagerTest, UpdatePasswordReencryptEverythingFillMetadata) {
 // ReEncryption, entity is not marked as unsynced.
 TEST_F(SyncManagerTest,
        UpdatePasswordReencryptEverythingDontMarkUnsyncWhenNotNeeded) {
-  base::FieldTrialList field_trial_list(nullptr);
-
   EXPECT_TRUE(SetUpEncryption(WRITE_TO_NIGORI, DEFAULT_ENCRYPTION));
   sync_pb::EntitySpecifics entity_specifics;
   {