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/policy.h b/policy_manager/policy.h
index 7554824..762b9a5 100644
--- a/policy_manager/policy.h
+++ b/policy_manager/policy.h
@@ -11,10 +11,10 @@
 namespace chromeos_policy_manager {
 
 // The three different results of a policy request.
-enum EvalStatus {
-  EvalStatusFailed,
-  EvalStatusSucceeded,
-  EvalStatusAskMeAgainLater,
+enum class EvalStatus {
+  kFailed,
+  kSucceeded,
+  kAskMeAgainLater,
 };
 
 // The Policy class is an interface to the ensemble of policy requests that the
@@ -31,8 +31,8 @@
   // first argument, a State instance, a returned error message, a returned
   // value and optionally followed by one or more arbitrary constant arguments.
   //
-  // When the implementation fails, the method returns EvalStatusFailed and sets
-  // the |error| string.
+  // When the implementation fails, the method returns EvalStatus::kFailed and
+  // sets the |error| string.
 
   // UpdateCheckAllowed returns whether it is allowed to request an update check
   // to Omaha.