shill: tighten up constness of PropertyStore

BUG=chromium-os:20543
TEST=unittest

Change-Id: Ifa86f82a883b3c3ec61ce10f8d82564b9a036b04
Reviewed-on: http://gerrit.chromium.org/gerrit/7901
Tested-by: mukesh agrawal <quiche@chromium.org>
Reviewed-by: Chris Masone <cmasone@chromium.org>
diff --git a/cellular_service.cc b/cellular_service.cc
index e2de6b7..26556d3 100644
--- a/cellular_service.cc
+++ b/cellular_service.cc
@@ -27,7 +27,7 @@
       strength_(0),
       cellular_(device),
       type_(flimflam::kTypeCellular) {
-  PropertyStore *store = this->store();
+  PropertyStore *store = this->mutable_store();
   store->RegisterConstString(flimflam::kActivationStateProperty,
                              &activation_state_);
   store->RegisterStringmap(flimflam::kCellularApnProperty, &apn_info_);