Use shell=False on non-Windows for gn-format.py

I don't exactly know why, but it wouldn't work for me on Mac with
shell=True. (I guess there must be few users other than me as no one
else noticed I guess.)

R=thakis@chromium.org

Review-Url: https://codereview.chromium.org/2296633003
Cr-Original-Commit-Position: refs/heads/master@{#415416}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 38182d4aa014f32cb1298a230514dea181b1b1c0
1 file changed
tree: f859008faaf88f5d89d00d85293a6dce61265697
  1. tools/