dbus: Rewrite ObjectProxy::CallMethodWithErrorCallback's comment

To make it clear which callback can get a nullptr when.

BUG=None

Review-Url: https://codereview.chromium.org/2299853002
Cr-Original-Commit-Position: refs/heads/master@{#415890}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: fd6b5515d45ca8559708fa1479dd762ad7c67225
1 file changed