libchromeos: Replace scoped_ptr with std::unique_ptr in D-Bus method handlers

Changed callbacks to use std::unique_ptr in D-Bus method handlers instead of
scoped_ptr, now that we can do this with base::Callback.
This eliminates the need to mix scoped_ptr and std::unique_ptr in the same
translation unit.

Sweep the code in the rest of platform2 that used libchromeos's D-Bus framework.

BUG=None
TEST=`FEATURES=test emerge-link libchromeos apmanager attestation buffet chromeos-dbus-bindings`

Change-Id: I50289198ef9ab183d5bc7d0c8cc7a8f53085e5a4
Reviewed-on: https://chromium-review.googlesource.com/267337
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Trybot-Ready: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: Christopher Wiley <wiley@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>

buffet/manager.cc

diff --git a/buffet/manager.cc b/buffet/manager.cc
index 33b1763..7b710f1 100644
--- a/buffet/manager.cc
+++ b/buffet/manager.cc
@@ -210,7 +210,7 @@
 }
 
 void Manager::SetCommandVisibility(
-    scoped_ptr<chromeos::dbus_utils::DBusMethodResponse<>> response,
+    std::unique_ptr<chromeos::dbus_utils::DBusMethodResponse<>> response,
     const std::vector<std::string>& in_names,
     const std::string& in_visibility) {
   CommandDefinition::Visibility visibility;