libmetrics: Add support for building against libchrome-271506

BUG=chromium:375032
CQ-DEPEND=CL:200532
CQ-DEPEND=CL:200522
TEST=`FEATURES=test emerge-$BOARD platform2`
TEST=Trybot runs on paladin, release, and chromiumos-sdk builders.

Change-Id: Ie3e9cc522099a8225f54a4442d0215d580425846
Reviewed-on: https://chromium-review.googlesource.com/200518


Reviewed-by: Daniel Erat <derat@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>