metrics: Fix upload_service to work with base::MessageLoop

With an earlier change, metrics_daemon was switched from the glib message loop
to base::MessageLoop.  UploadService still is trying to interact with the
glib message loop, it needs to be switched to using base::MessageLoop.

BUG=chromium:452228
TEST=`FEATURES=test emerge-panther metrics`

Change-Id: I38eb52ca1995d75cfb7d0e69a434e649493e7c6f
Reviewed-on: https://chromium-review.googlesource.com/243429


Reviewed-by: Nathan Bullock <nathanbullock@google.com>
Reviewed-by: Bertrand Simonnet <bsimonnet@chromium.org>
Tested-by: Stephen Fung <stevefung@chromium.org>
Commit-Queue: Stephen Fung <stevefung@chromium.org>
Trybot-Ready: Stephen Fung <stevefung@chromium.org>