metrics: Migrate libchromeos to its own ebuild

All platform2 packages should have their own ebuild that will be compiled
independently. Packages should DEPEND on libchromeos and should not have a gyp
dependency on libchromeos's gyp file anymore.

BUG=chromium:381372
TEST=emerge-daisy libchromeos && emerge-daisy platform2 work
TEST=trybot run on daisy, link, mario and duck.
TEST=trybot run on lumpy-incremental-paladin.

CQ-DEPEND=CL:202748

Change-Id: I0fe0732d47463e880b11d3d547e99dba0ac83ace
Reviewed-on: https://chromium-review.googlesource.com/202771


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