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>