Add an uploader to metrics_daemon

Metrics_daemon will now upload the metrics to Chrome's backend when Chrome is
not available.
This uses //components/metrics from chrome to use its protobuf definition and
the metrics common code.

This functionality is not yet enabled. It will be once the end-to-end test is
enabled.

BUG=chromium:358283, chromium:364579
TEST=FEATURES=test emerge-amd64-generic metrics

CQ-DEPEND=CL:205790
CQ-DEPEND=CL:206055

Change-Id: I87aaf7a2ac041581fa3ffd4ec61f73e933c00a52
Reviewed-on: https://chromium-review.googlesource.com/205810


Reviewed-by: Bertrand Simonnet <bsimonnet@chromium.org>
Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>
Reviewed-by: Luigi Semenzato <semenzato@chromium.org>
Commit-Queue: Bertrand Simonnet <bsimonnet@chromium.org>