PolicyManager: Move payload_size to int64_t.

Code style encourages signed types instead of unsigned types and the
value being exposed is already signed. This patch fixes that for the
payload_size variable and adapts the BoxedValue::ValuePrinter
implementations to use the int64_t and uint64_t types.

BUG=None
TEST=Unit tests updated

Change-Id: I21310c59d8c2654c43cac27265055c8577341562
Reviewed-on: https://chromium-review.googlesource.com/198269
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/updater_provider.h b/policy_manager/updater_provider.h
index fa5f48a..ff76cce 100644
--- a/policy_manager/updater_provider.h
+++ b/policy_manager/updater_provider.h
@@ -59,8 +59,9 @@
   // A variable returning the update target version.
   virtual Variable<std::string>* var_new_version() = 0;
 
-  // A variable returning the update payload size.
-  virtual Variable<size_t>* var_payload_size() = 0;
+  // A variable returning the update payload size. The payload size is
+  // guaranteed to be non-negative.
+  virtual Variable<int64_t>* var_payload_size() = 0;
 
   // A variable returning the current channel.
   virtual Variable<std::string>* var_curr_channel() = 0;