Currently check_bucket_permissions() in download_from_google_storage.py
performs a gsutil ls gs://bucket to determine whether the user has access
to the bucket or not.
This can be an EXTREMELY expensive operation (~minutes) if the bucket in
question has a lot of objects in the root (real case: chrome-telemetry).
It is worth noting that check_bucket_permissions() is not called just for
uploads but also for downloads, hence this is slowing down all invocations
of gclient sync on users and bots machine.
Removing the check_bucket_permissions() and let gsutil fail with an
Unauthorized error message if ACLs are not met.

BUG=458059

Review URL: https://codereview.chromium.org/923473002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@294083 0039d316-1c4b-4281-b951-d872f2087c98