Disabling Flaky Test RollbackUnavailable

ResetFirstAfterBootTest.RollbackUnavailable has been flaky. Disabling.

TBR=xiyuan@chromium.org

Bug: 870284
Change-Id: I7226ae73ca9c4cf440de3fbeae28886309d907d9
Reviewed-on: https://chromium-review.googlesource.com/1160866Reviewed-by: Tommy Martino <tmartino@chromium.org>
Commit-Queue: Tommy Martino <tmartino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580207}

chrome/browser/chromeos/login/reset_browsertest.cc

@@ -216,7 +216,8 @@ IN_PROC_BROWSER_TEST_F(ResetFirstAfterBootTest, PRE_RollbackUnavailable) {
   RegisterSomeUser();
 }
 
-IN_PROC_BROWSER_TEST_F(ResetFirstAfterBootTest, RollbackUnavailable) {
+// Disabled due to flakiness (crbug.com/870284)
+IN_PROC_BROWSER_TEST_F(ResetFirstAfterBootTest, DISABLED_RollbackUnavailable) {
   update_engine_client_->set_can_rollback_check_result(false);
 
   InvokeResetScreen();