Ensure GYP_MSVS_VERSION is set for GN build.

build/vs_toolchain.py

 #!/usr/bin/env python
 # Copyright 2014 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.
 
 import json
 import os
 import pipes
 import shutil
 import subprocess
@@ skipping 89 matching lines @@
 
 def DetectVisualStudioPath():
   """Return path to the GYP_MSVS_VERSION of Visual Studio.
   """
 
   # Note that this code is used from
   # build/toolchain/win/setup_toolchain.py as well.
 
   # Default to Visual Studio 2013 for now.
   version_as_year = os.environ.get('GYP_MSVS_VERSION', '2013')
+  os.environ['GYP_MSVS_VERSION'] = version_as_year

[Daniel Bratell, 2016/01/12 08:42:02]

From a code maintenance point of view it doesn't seem right that the "Detect"
function has side effects on the environment. 

What do you think about moving it to _SetupScript in setup_toolchain.py and to
SetEnvironmentAndGetRuntimeDllDirs in vs_toolchain.py, or to a new shared helper
function?

   year_to_version = {
       '2013': '12.0',
       '2015': '14.0',
   }
   if version_as_year not in year_to_version:
     raise Exception(('Visual Studio version %s (from GYP_MSVS_VERSION)'
                      ' not supported. Supported versions are: %s') % (
                        version_as_year, ', '.join(year_to_version.keys())))
   version = year_to_version[version_as_year]
   keys = [r'HKLM\Software\Microsoft\VisualStudio\%s' % version,
@@ skipping 205 matching lines @@
       'copy_dlls': CopyDlls,
   }
   if len(sys.argv) < 2 or sys.argv[1] not in commands:
     print >>sys.stderr, 'Expected one of: %s' % ', '.join(commands)
     return 1
   return commands[sys.argv[1]](*sys.argv[2:])
 
 
 if __name__ == '__main__':
   sys.exit(main())