PolicyManager: New PRNG to use together with the RandomProvider's seed.

The RandomProvider gives you a single random value that you can use
to generate more unsecure random numbers on policies in a
deterministic way.

BUG=chromium:358269
TEST=Unittest added.

Change-Id: Ie344014c55cc56e7dbdf3ce679eb3ca37be52678
Reviewed-on: https://chromium-review.googlesource.com/197441
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 fda628c..39842ea 100644
--- a/SConstruct
+++ b/SConstruct
@@ -305,6 +305,7 @@
                             policy_manager/generic_variables_unittest.cc
                             policy_manager/pmtest_utils.cc
                             policy_manager/policy_manager_unittest.cc
+                            policy_manager/prng_unittest.cc
                             policy_manager/real_config_provider_unittest.cc
                             policy_manager/real_device_policy_provider_unittest.cc
                             policy_manager/real_random_provider_unittest.cc