gsutil.py is cumbersome to use on Windows, unlike on Linux and Mac. This
change adds a batch file like the ones for gclient and other tools.

Bug: https://github.com/dart-lang/sdk/issues/35367
Change-Id: Ib33dc27573de5485ae93b525002f5fbcb3a5f4d1
Reviewed-on: https://chromium-review.googlesource.com/c/1370124
Commit-Queue: Jonas Termansen <sortie@google.com>
Reviewed-by: Ryan Tseng <hinoka@chromium.org>