Remove usages of BASE_VER and LIBCHROME_VERS in overlay.

BUG=chromium:1062979
TEST=CQ

Change-Id: I1a54ccea585cbd41c36f76a93c3171738b84bfc6
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/chromiumos-overlay/+/2277912
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Tested-by: Qijiang Fan <fqj@google.com>
Commit-Queue: Qijiang Fan <fqj@google.com>
7 files changed