update_engine: Fix bug in rollback tests

This change fixes a bug identified in the asan tests where the
value of rollback_allowed_milestones int he fake request_params
was not intialized properly in a testcase, causing a failure.

BUG=chromium:935140
TEST=update engine unittests w/ asan profile
Change-Id: Iabf5ba126f6155caa679f43b29472c353e6d308f
Reviewed-on: https://chromium-review.googlesource.com/1488192
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Bailey Berro <baileyberro@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
diff --git a/omaha_request_action_unittest.cc b/omaha_request_action_unittest.cc
index 66fc6fe..a642b5a 100644
--- a/omaha_request_action_unittest.cc
+++ b/omaha_request_action_unittest.cc
@@ -3187,6 +3187,7 @@
 
 TEST_F(OmahaRequestActionTest, PastRollbackVersionsValidEntries) {
   OmahaResponse response;
+  request_params_.set_rollback_allowed_milestones(4);
   fake_update_response_.rollback = true;
   fake_update_response_.rollback_allowed_milestones = 4;
   fake_update_response_.rollback_firmware_version = "4.3";