PM: New (stub) policy for whether an update can be applied.

There's no implementation for this policy yet, and so it always returns
true.

BUG=chromium:358323
TEST=Unit tests.

Change-Id: Id3a0d13cf8c2e9061b800b114d8476c11f998df0
Reviewed-on: https://chromium-review.googlesource.com/197376
Reviewed-by: Gilad Arnold <garnold@chromium.org>
Commit-Queue: Gilad Arnold <garnold@chromium.org>
Tested-by: Gilad Arnold <garnold@chromium.org>
diff --git a/policy_manager/policy.h b/policy_manager/policy.h
index 5896fe7..7c7ec0c 100644
--- a/policy_manager/policy.h
+++ b/policy_manager/policy.h
@@ -40,6 +40,12 @@
                                         std::string* error,
                                         bool* result) const = 0;
 
+  // Returns whether an update can be downloaded/applied.
+  virtual EvalStatus UpdateDownloadAndApplyAllowed(EvaluationContext* ec,
+                                                   State* state,
+                                                   std::string* error,
+                                                   bool* result) const = 0;
+
  protected:
   Policy() {}