Revert "Sync: don't start up again (in transport mode) after StopAndClear"

This reverts commit 42484004.

Reason for revert: Investigation has concluded, see crbug.com/1120899

Original change's description:
> Sync: don't start up again (in transport mode) after StopAndClear
>
> This is essentially a revert of crrev.com/c/1082267 (undoing the actual
> 1-line change from that CL, but keeping the comment improvements).
> This is a temporary measure to investigate crbug.com/1120899.
>
> Bug: 1120899, 1082267
> Change-Id: I7761418c038824218a7fc36b325f1b364f7bbbe5
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2401460
> Commit-Queue: Marc Treib <treib@chromium.org>
> Reviewed-by: Mikel Astiz <mastiz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#805489}

TBR=treib@chromium.org,mastiz@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 1120899
Bug: 1082267
Change-Id: I75e64d1988cc585e2aa344058ac4d50dbb73fd34
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2438386Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812035}

components/sync/driver/profile_sync_service.cc

@@ -60,9 +60,6 @@ namespace syncer {
 
 namespace {
 
-constexpr base::Feature kSyncRestartAfterStopAndClear{
-    "SyncRestartAfterStopAndClear", base::FEATURE_DISABLED_BY_DEFAULT};
-
 // The initial state of sync, for the Sync.InitialState histogram. Even if
 // this value is CAN_START, sync startup might fail for reasons that we may
 // want to consider logging in the future, such as a passphrase needed for
@@ -1876,12 +1873,8 @@ void ProfileSyncService::StopAndClear() {
   // away or it treats all "Cancel the confirmation" cases?
   if (!user_settings_->IsSyncRequested()) {
     StopImpl(CLEAR_DATA);
-    // TODO(crbug.com/1120899): Doing this might cause some clients to enter a
-    // persistent error state.
-    if (base::FeatureList::IsEnabled(kSyncRestartAfterStopAndClear)) {
-      // Try to start up again (in transport-only mode).
-      startup_controller_->TryStart(/*force_immediate=*/true);
-    }
+    // Try to start up again (in transport-only mode).
+    startup_controller_->TryStart(/*force_immediate=*/true);
     return;
   }
 

components/sync/driver/profile_sync_service_startup_unittest.cc

@@ -413,8 +413,7 @@ TEST_F(ProfileSyncServiceStartupTest, DisableSync) {
   // mode. It should immediately start up again in transport mode.
   SetUpFakeSyncEngine();
   data_type_manager = SetUpDataTypeManagerMock();
-  // TODO(crbug.com/1082267, crbug.com/1120899): Reinstate this expectation.
-  // EXPECT_CALL(*data_type_manager, Configure(_, _));
+  EXPECT_CALL(*data_type_manager, Configure(_, _));
   sync_service()->StopAndClear();
 }