PolicyManager: Convert EvalStatus to a enum class.

This simple patch converts EvalStatus to a enum class to have better
type checking over the enum and group the constants on a namespace.

BUG=chromium:340871
TEST=Build and run unit tests.

Change-Id: I56e8d52325c5de578a6c78c7c85c9dccf61d48a1
Reviewed-on: https://chromium-review.googlesource.com/189635
Reviewed-by: Alex Deymo <deymo@chromium.org>
Commit-Queue: Alex Deymo <deymo@chromium.org>
Tested-by: Alex Deymo <deymo@chromium.org>
diff --git a/policy_manager/chromeos_policy.cc b/policy_manager/chromeos_policy.cc
index 95578aa..cd10918 100644
--- a/policy_manager/chromeos_policy.cc
+++ b/policy_manager/chromeos_policy.cc
@@ -15,7 +15,7 @@
                                               bool* result) const {
   // TODO(deymo): Write this policy implementation with the actual policy.
   *result = true;
-  return EvalStatusSucceeded;
+  return EvalStatus::kSucceeded;
 }
 
 }  // namespace chromeos_policy_manager