cbuildbot: Ensure incremental builders do not clear chroot midrun.

BUG=chromium:584035
TEST=sync_stages_unittest still passes

Change-Id: I66e0ff3b6363d9e93ed03fe7d58d2b2ea89175a0
Reviewed-on: https://chromium-review.googlesource.com/325824
Commit-Ready: Kevin Cheng <kevcheng@chromium.org>
Tested-by: Kevin Cheng <kevcheng@chromium.org>
Reviewed-by: Kevin Cheng <kevcheng@chromium.org>
1 file changed