Add a --force flag to get_toolchain_if_necessary.py to let it run on non-Windows.

win_toolchain/get_toolchain_if_necessary.py

Index: win_toolchain/get_toolchain_if_necessary.py
diff --git a/win_toolchain/get_toolchain_if_necessary.py b/win_toolchain/get_toolchain_if_necessary.py
index 7187bf491b71fb7860715737d4dac64bfd2b8439..c38785d1732e1eef7c8ff65f4095b545730effe2 100755
--- a/win_toolchain/get_toolchain_if_necessary.py
+++ b/win_toolchain/get_toolchain_if_necessary.py
@@ -213,14 +213,16 @@ def DoTreeMirror(target_dir, tree_sha1):
 
 
 def main():
-  if not sys.platform.startswith(('cygwin', 'win32')):
-    return 0
-
   parser = optparse.OptionParser(description=sys.modules[__name__].__doc__)
   parser.add_option('--output-json', metavar='FILE',
                     help='write information about toolchain to FILE')
+  parser.add_option('--force', action='store_true',
+                    help='force script to run on non-Windows hosts')
   options, args = parser.parse_args()
 
+  if not (sys.platform.startswith(('cygwin', 'win32')) or options.force):
+    return 0
+
   if sys.platform == 'cygwin':
     # This script requires Windows Python, so invoke with depot_tools' Python.
     def winpath(path):