update_engine: qualify stl_util functions

This uses ContainsKey from stl_util which has been moved to the
base namespace in future versions of libchrome.

BUG=b:37434548
TEST=emerge, run unit tests

Change-Id: Id81f7ba125ce99f5a95af317ecfa8910e740dd22
Reviewed-on: https://chromium-review.googlesource.com/882383
Commit-Ready: Eric Caruso <ejcaruso@chromium.org>
Tested-by: Eric Caruso <ejcaruso@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
diff --git a/connection_manager.cc b/connection_manager.cc
index c225cb2..ca173ef 100644
--- a/connection_manager.cc
+++ b/connection_manager.cc
@@ -86,7 +86,7 @@
       if (device_policy->GetAllowedConnectionTypesForUpdate(&allowed_types)) {
         // The update setting is enforced by the device policy.
 
-        if (!ContainsKey(allowed_types, shill::kTypeCellular)) {
+        if (!base::ContainsKey(allowed_types, shill::kTypeCellular)) {
           LOG(INFO) << "Disabling updates over cellular per device policy.";
           return false;
         }