This CL makes fetch default to sso:// within the cog env. It also adds a helper within gclient_scm to check if the env is cog.

Bug: chrome-operations:170
Change-Id: I8c35348653406549fadbd08df3d9991bb24ca776
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3670721Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Joanna Wang <jojwang@chromium.org>
Commit-Queue: Aravind Vasudevan <aravindvasudev@google.com>