Reenable ResetFirstAfterBootTest.RollbackUnavailable

The failure due to which the test got disabled seems like an issue in
blink, and suspected root cause has been fixed.

BUG= 870284

Change-Id: Ia177fc087231a8fc298a1f3900b56e378dbabc3f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1566679
Auto-Submit: Toni Baržić <tbarzic@chromium.org>
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Commit-Queue: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/master@{#650580}

chrome/browser/chromeos/login/reset_browsertest.cc

@@ -272,8 +272,7 @@ IN_PROC_BROWSER_TEST_F(ResetFirstAfterBootTest, PRE_RollbackUnavailable) {
   RegisterSomeUser();
 }
 
-// Disabled due to flakiness (crbug.com/870284)
-IN_PROC_BROWSER_TEST_F(ResetFirstAfterBootTest, DISABLED_RollbackUnavailable) {
+IN_PROC_BROWSER_TEST_F(ResetFirstAfterBootTest, RollbackUnavailable) {
   InvokeResetScreen();
   EXPECT_EQ(0, FakePowerManagerClient::Get()->num_request_restart_calls());
   EXPECT_EQ(0, FakeSessionManagerClient::Get()->start_device_wipe_call_count());