Store remaining usage time in milliseconds

It was stored in minutes, which was causing some rounding problems when the
CheckTimeLimit function was called less than 1 minute before the device being
locked by a screen time.

Bug: 895036
Change-Id: Ia6d243e8d59e32fa07151e76542b6235caf5b33a
Reviewed-on: https://chromium-review.googlesource.com/c/1281662
Commit-Queue: Henrique Grandinetti <hgrandinetti@google.com>
Commit-Queue: Rahul Chaturvedi <rkc@chromium.org>
Reviewed-by: Rahul Chaturvedi <rkc@chromium.org>
Reviewed-by: Aga Wronska <agawronska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599747}

chrome/browser/chromeos/child_accounts/screen_time_controller.cc

@@ -242,7 +242,8 @@ void ScreenTimeController::SaveCurrentStateToPref(
   state_dict->SetKey(kScreenStateTimeUsageLimitEnabled,
                      base::Value(state.is_time_usage_limit_enabled));
   state_dict->SetKey(kScreenStateRemainingUsage,
-                     base::Value(state.remaining_usage.InMinutes()));
+                     base::Value(base::checked_cast<int>(
+                         state.remaining_usage.InMilliseconds())));
   state_dict->SetKey(kScreenStateUsageLimitStarted,
                      base::Value(state.time_usage_limit_started.ToDoubleT()));
   state_dict->SetKey(kScreenStateNextStateChangeTime,
@@ -300,7 +301,7 @@ ScreenTimeController::GetLastStateFromPref() {
   if (!remaining_usage || !remaining_usage->is_int())
     return base::nullopt;
   result.remaining_usage =
-      base::TimeDelta::FromMinutes(remaining_usage->GetInt());
+      base::TimeDelta::FromMilliseconds(remaining_usage->GetInt());
 
   // Verify time_usage_limit_started from the pref is a double value.
   const base::Value* time_usage_limit_started =