power: libchrome uprev

Pass by reference instead of copy in for loop in
DBusWrapperStub::NotifyServiceAvailable.
In new libchrome, WaitForServiceToBeAvailableCallback changed from
base::Callback to base::OnceCallback which is uncopyable.

BUG=chromium:909719
TEST=run unit tests

Change-Id: I2106c14a9c8bbb3ecdbf0c46e275bff90f354044
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform2/+/1980442
Tested-by: Grace Cham <hscham@chromium.org>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Eric Caruso <ejcaruso@chromium.org>
Reviewed-by: Qijiang Fan <fqj@google.com>
Commit-Queue: Grace Cham <hscham@chromium.org>
diff --git a/power_manager/powerd/system/dbus_wrapper_stub.cc b/power_manager/powerd/system/dbus_wrapper_stub.cc
index 24c577a..430d7c5 100644
--- a/power_manager/powerd/system/dbus_wrapper_stub.cc
+++ b/power_manager/powerd/system/dbus_wrapper_stub.cc
@@ -149,7 +149,7 @@
   if (it == service_availability_callbacks_.end())
     return;
 
-  for (auto cb : it->second)
+  for (auto& cb : it->second)
     std::move(cb).Run(available);
   service_availability_callbacks_.erase(it);
 }