Add windows .bat file for gsutil.py.

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>

gsutil.py.bat

@@ -0,0 +1,15 @@
+@echo off
+:: Copyright (c) 2018 The Chromium Authors. All rights reserved.
+:: Use of this source code is governed by a BSD-style license that can be
+:: found in the LICENSE file.
+setlocal
+
+:: Synchronize the root directory before deferring control back to gsutil.py.
+call "%~dp0update_depot_tools.bat" %*
+
+:: Ensure that "depot_tools" is somewhere in PATH so this tool can be used
+:: standalone, but allow other PATH manipulations to take priority.
+set PATH=%PATH%;%~dp0
+
+:: Defer control.
+python "%~dp0gsutil.py" %*