Update gyp files to depend on metrics' standalone ebuild.

Metrics is now installed by chromeos-base/metrics.
Platform2 packages should depend on it through the variable:deps mechanism
instead of depending directly on metrics' gyp file.

BUG=chromium:387923
TEST=Build platform2. metrics gets installed and the tests succeed.
TEST=Trybot run on daisy-release, lumpy-incremental-paladin, link-release,
  duck-release, x86-mario-release.

CQ-DEPEND=CL:205312

Change-Id: I07607d5fa574ce3c273fc4189590453edbee2aa4
Reviewed-on: https://chromium-review.googlesource.com/205299


Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>
Commit-Queue: Bertrand Simonnet <bsimonnet@chromium.org>