dbus::ScopedDBusError has inline constructor/destructor and
is_set() method that call into libdbus-1. By placing these
function calls in a header file, we add implicit link dependency
on components that need to use ScopedDBusError. If we move
them out to a .cc file, the dependency will be localized to
libchrome.

BUG=chromium:416628
TEST=dbus_unittests pass

Review URL: https://codereview.chromium.org/592793004

Cr-Commit-Position: refs/heads/master@{#296478}