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/default_profile_unittest.cc b/default_profile_unittest.cc
index e819185..84ecc16 100644
--- a/default_profile_unittest.cc
+++ b/default_profile_unittest.cc
@@ -63,9 +63,10 @@
   {
     Error error(Error::kInvalidProperty, "");
     EXPECT_FALSE(
-        profile_->store()->SetBoolProperty(flimflam::kOfflineModeProperty,
-                                           true,
-                                           &error));
+        profile_->mutable_store()->SetBoolProperty(
+            flimflam::kOfflineModeProperty,
+            true,
+            &error));
   }
 }