PolicyManager: Add a generic copy variable.

This patch includes a generic class of Variable that creates copies
of a referenced object using the copy constructor. A provider that
needs to return the value of fixed object like a private member can
simply use this generic CopyVariable class.

BUG=None
TEST=unittest

Change-Id: I4e7703c0d25990f42c9c83228ba177baffadc194
Reviewed-on: https://chromium-review.googlesource.com/181963
Reviewed-by: Alex Deymo <deymo@chromium.org>
Tested-by: Alex Deymo <deymo@chromium.org>
Commit-Queue: Alex Deymo <deymo@chromium.org>
diff --git a/SConstruct b/SConstruct
index 2e1a155..7e0c30b 100644
--- a/SConstruct
+++ b/SConstruct
@@ -330,6 +330,7 @@
                             p2p_manager_unittest.cc
                             payload_signer_unittest.cc
                             payload_state_unittest.cc
+                            policy_manager/generic_variables_unittest.cc
                             postinstall_runner_action_unittest.cc
                             prefs_unittest.cc
                             simple_key_value_store_unittest.cc