PolicyManager: Move the global variables to Provider members.

Per the Google C++ Style Guide, the global variables can't be
instances of a class and that includes smart pointers like
scoped_ptr. The alternative is to use plain pointers such as a
Variable<T>*. The disadvantage of this approach is that the memory
pointed by this global variable is managed by a singleton class
(a FooProvider) on production code and a different class during
testing (a FakeFooProvider). Also, these classes, fake and real
implementation, aren't forced to implement the same set of variables
since there's no common interface. Finally, the
construction/destruction of these variables also needs to handle the
global pointer, nulifying it after deletion.

This patch moves the global variable to a member of a base class that
hides the initialization of these variables to a subclass. This patch
includes a subclass with the real implementation.

BUG=chromium:338591
TEST=unittest passes.

Change-Id: I0aa43c51d72c65d97be280fd58307d085bb8577b
Reviewed-on: https://chromium-review.googlesource.com/184774
Tested-by: Alex Deymo <deymo@chromium.org>
Reviewed-by: Gilad Arnold <garnold@chromium.org>
Commit-Queue: Alex Deymo <deymo@chromium.org>
diff --git a/SConstruct b/SConstruct
index 0d121d5..4d4003a 100644
--- a/SConstruct
+++ b/SConstruct
@@ -282,8 +282,7 @@
                    p2p_manager.cc
                    payload_signer.cc
                    payload_state.cc
-                   policy_manager/random_provider.cc
-                   policy_manager/random_vars.cc
+                   policy_manager/real_random_provider.cc
                    postinstall_runner_action.cc
                    prefs.cc
                    proxy_resolver.cc
@@ -333,7 +332,7 @@
                             payload_signer_unittest.cc
                             payload_state_unittest.cc
                             policy_manager/generic_variables_unittest.cc
-                            policy_manager/random_provider_unittest.cc
+                            policy_manager/real_random_provider_unittest.cc
                             policy_manager/variable_unittest.cc
                             postinstall_runner_action_unittest.cc
                             prefs_unittest.cc