Enable compiling with automatically-downloaded VS toolchain on Windows bots

make.py

 # Copyright 2011 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # "Makefile" replacement to build skia for Windows.
 # More info at https://sites.google.com/site/skiadocs/
 #
 # Some usage examples:
 #   make clean
@@ skipping 74 matching lines @@
         print '\nUnable to find vcvars32.bat on your system.'
     sys.exit(1)
 
 
 def MakeWindows(targets):
     """For Windows: build as appropriate for the command line arguments.
 
     parameters:
         targets: build targets as a list of strings
     """
-    # TODO(epoger): I'm not sure if this is needed for ninja builds.
-    CheckWindowsEnvironment()
+    if os.environ.get('CHROME_HEADLESS', '0') != '1':
+        # TODO(epoger): I'm not sure if this is needed for ninja builds.
+        CheckWindowsEnvironment()
 
     # Run gyp_skia to prepare Visual Studio projects.
     cd(SCRIPT_DIR)
     runcommand('python gyp_skia')
 
     # We already built the gypfiles...
     while TARGET_GYP in targets:
         targets.remove(TARGET_GYP)
 
     # And call ninja to do the work!
@@ skipping 56 matching lines @@
         print 'unknown platform (os.name=%s, sys.platform=%s); see %s' % (
             os.name, sys.platform, 'https://sites.google.com/site/skiadocs/')
         sys.exit(1)
     sys.exit(0)
 
 
 # main()
 Make(sys.argv[1:])