wm: Stop linking libchromeos. We no longer need to link libchromeos to use D-Bus service constants. Including dbus/service_constants.h, provided by system_api, just suffices. BUG=chromium-os:19054 TEST=emerge-x86-alex chromeos-wm Change-Id: I7799e0381a0bebd0e27c4834b8ba4c2f48b5f299 Reviewed-on: http://gerrit.chromium.org/gerrit/6177 Tested-by: Satoru Takabayashi <[email protected]> Reviewed-by: Daniel Erat <[email protected]>