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/default_policy.h b/policy_manager/default_policy.h
index 20f6668..f5fe89d 100644
--- a/policy_manager/default_policy.h
+++ b/policy_manager/default_policy.h
@@ -22,7 +22,7 @@
                                         std::string* error,
                                         bool* result) const {
     *result = true;
-    return EvalStatusSucceeded;
+    return EvalStatus::kSucceeded;
   }
 
  private: