metrics: Activate the uploader for embedded boards

Now that the code is under tests, add an extra flag to metrics_daemon.conf to
enable the uploader when chrome is not available.

BUG=chromium:389222
TEST=build and deploy metrics on gizmo. metrics_daemon is running and the
metrics are reported.

CQ-DEPEND=CL:214286

Change-Id: I5daa72a95c3bf1c15387a40c6964be44bc6da082
Reviewed-on: https://chromium-review.googlesource.com/214285
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Bertrand Simonnet <bsimonnet@chromium.org>
Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>

metrics/init/metrics_daemon.conf

@@ -16,5 +16,8 @@ pre-start script
   mkdir -p /var/lib/metrics
 end script
 
+# metrics will update the next line to add -uploader for embedded builds.
+env DAEMON_FLAGS=""
+
 expect fork
-exec metrics_daemon
+exec metrics_daemon ${DAEMON_FLAGS}