PM: Add BoxedValue::ToString() method.

This will be used to dump variables in the EvaluationContext.

BUG=chromium:355724
TEST=New unit test + Unit tests pass.

Change-Id: I82a3fb5d2636fd124dfbf63bf130fcd3c5726bcd
Reviewed-on: https://chromium-review.googlesource.com/197524
Reviewed-by: Alex Deymo <deymo@chromium.org>
Commit-Queue: David Zeuthen <zeuthen@chromium.org>
Tested-by: David Zeuthen <zeuthen@chromium.org>
diff --git a/SConstruct b/SConstruct
index 2fb23bc..7730995 100644
--- a/SConstruct
+++ b/SConstruct
@@ -237,6 +237,7 @@
                    p2p_manager.cc
                    payload_signer.cc
                    payload_state.cc
+                   policy_manager/boxed_value.cc
                    policy_manager/chromeos_policy.cc
                    policy_manager/evaluation_context.cc
                    policy_manager/event_loop.cc