metrics: Import protobufs from Chromium.

Instead of pulling the protobufs from chromium, copy them into
platform2/metrics/proto.
This enables us to add Chromium OS specific changes without polluting
Chromium's protobuf.

BUG=chrome-os-partner:42861
TEST=`FEATURES=test emerge-gizmo metrics` succeeds.

CQ-DEPEND=CL:286895

Change-Id: Ib919a21da6518199c29adb70c8bc27220e0dc031
Reviewed-on: https://chromium-review.googlesource.com/286943


Reviewed-by: Gaurav Shah <gauravsh@chromium.org>
Commit-Queue: Bertrand Simonnet <bsimonnet@chromium.org>
Trybot-Ready: Bertrand Simonnet <bsimonnet@chromium.org>
Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>