The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform/bisect-kit/+/a99eb4d9a070c707d9ad44b5825d51f4f1a243e1

commit a99eb4d9a070c707d9ad44b5825d51f4f1a243e1
Author: Kuang-che Wu <kcwu@chromium.org>
Date: Tue Oct 02 12:59:15 2018

bisect-kit: chromeos sdk uses chroot image for better performance

On btrfs, build_packages time is significant slower without chroot image
(4900s vs 1800s; with tarball cache and ccache cache). I guess the
reason is because emerge generates tons of files to temp folder and
deletes them after build, and btrfs has known poor performance on
deleting files.

There is no performance difference on ext4fs. So this CL always uses
chroot image.

BUG=chromium:878698
TEST=switch_cros_cr_localbuild_internal.py with btrfs and observe the
performance

Change-Id: Iede06d332a426eb4c3b0363b57dabe999ce91caa
Reviewed-on: https://chromium-review.googlesource.com/1253861
Commit-Ready: Kuang-che Wu <kcwu@chromium.org>
Tested-by: Kuang-che Wu <kcwu@chromium.org>
Reviewed-by: Chi-Ngai Wan <cnwan@google.com>

[modify] https://crrev.com/a99eb4d9a070c707d9ad44b5825d51f4f1a243e1/switch_cros_localbuild.py