crosperf: Add keep_stateful experiment field

Experiment field "keep_stateful=true" flags the image_chromeos script to
drop "--clobber-stateful" flag in `cros flash` which was always on
before.
This is especially helpful when running tests on a DUT connected via
Wi-Fi. There are also other uses cases where we want to keep the
stateful information.

BUG=b:277269271
TEST=tested locally with crosperf

Change-Id: I11bf25a4c84ac6dce0abbf04a340e1bca6007f6f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/4484929
Reviewed-by: George Burgess <gbiv@chromium.org>
Tested-by: Denis Nikitin <denik@chromium.org>
Commit-Queue: Denis Nikitin <denik@chromium.org>
8 files changed