cros flash: static_dir should not be owned by root
cros flash: static_dir should not be owned by root

cros flash needs to be able to write to devserver's static_dir to
create symlinks for local image paths. This CL makes sure we create
static_dir as a non-root user.

BUG=chromium:335429
TEST=tested with root-owned static_dir

Change-Id: Ic71c8167038bfcd8a755fa9c36ac06a0c2906ec7
Reviewed-on: https://chromium-review.googlesource.com/182935
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Commit-Queue: Yu-Ju Hong <52dc720c411f8e2156e608c1a8b894a917489667@chromium.org>
Tested-by: Yu-Ju Hong <52dc720c411f8e2156e608c1a8b894a917489667@chromium.org>

bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite