PolicyManager: Add an base Variable class without type.

The Variable<T> is a parametric type that requires to know the type T
of the exported variable. This new BaseVariable class isn't
parametric and allows to keep a list of variable pointers on the
EvaluationContext to cache their provided results and dump the names
of those variables.

BUG=chromium:338590
TEST=unittest passes.

Change-Id: I1677e3975d44575ed12f35c36381101d4379c5fc
Reviewed-on: https://chromium-review.googlesource.com/184428
Tested-by: Alex Deymo <deymo@chromium.org>
Reviewed-by: Alex Deymo <deymo@chromium.org>
Commit-Queue: Alex Deymo <deymo@chromium.org>
diff --git a/SConstruct b/SConstruct
index df6e8d8..0d121d5 100644
--- a/SConstruct
+++ b/SConstruct
@@ -334,6 +334,7 @@
                             payload_state_unittest.cc
                             policy_manager/generic_variables_unittest.cc
                             policy_manager/random_provider_unittest.cc
+                            policy_manager/variable_unittest.cc
                             postinstall_runner_action_unittest.cc
                             prefs_unittest.cc
                             simple_key_value_store_unittest.cc