buffet: Replace CallbackList with vector<Callback>

We don't remove callback at runtime, so callbacks on WeakPtr
are enough there.

BUG=brillo:697
TEST=FEATURE=test emerge-gizmo buffet

Change-Id: I02573979b208d0e9df62e59f4bb3bbec92608b71
Reviewed-on: https://chromium-review.googlesource.com/270279
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>
Tested-by: Vitaly Buka <vitalybuka@chromium.org>
diff --git a/buffet/commands/command_manager_unittest.cc b/buffet/commands/command_manager_unittest.cc
index 5b45cee..76a3246 100644
--- a/buffet/commands/command_manager_unittest.cc
+++ b/buffet/commands/command_manager_unittest.cc
@@ -164,7 +164,7 @@
   CommandManager manager;
   int update_count = 0;
   auto on_command_change = [&update_count]() { update_count++; };
-  auto token = manager.AddOnCommandDefChanged(base::Bind(on_command_change));
+  manager.AddOnCommandDefChanged(base::Bind(on_command_change));
 
   auto json = CreateDictionaryValue(R"({
     'foo': {