[gn] Default to ninja on all platforms

gypfiles/gyp_environment.py

 # Copyright 2015 the V8 project authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """
 Sets up various automatic gyp environment variables. These are used by
 gyp_v8 and landmines.py which run at different stages of runhooks. To
 make sure settings are consistent between them, all setup should happen here.
 """
 
@@ skipping 13 matching lines @@
     return
   file_contents = open(file_path).read()
   try:
     file_data = eval(file_contents, {'__builtins__': None}, None)
   except SyntaxError, e:
     e.filename = os.path.abspath(file_path)
     raise
   supported_vars = ( 'V8_GYP_FILE',
                      'V8_GYP_SYNTAX_CHECK',
                      'GYP_DEFINES',
+                     'GYP_GENERATORS',
                      'GYP_GENERATOR_FLAGS',
                      'GYP_GENERATOR_OUTPUT', )
   for var in supported_vars:
     val = file_data.get(var)
     if val:
       if var in os.environ:
         print 'INFO: Environment value for "%s" overrides value in %s.' % (
             var, os.path.abspath(file_path)
         )
       else:
         os.environ[var] = val
 
 
 def set_environment():
   """Sets defaults for GYP_* variables."""
 
   if 'SKIP_V8_GYP_ENV' not in os.environ:
     # Update the environment based on v8.gyp_env
     gyp_env_path = os.path.join(os.path.dirname(V8_ROOT), 'v8.gyp_env')
     apply_gyp_environment(gyp_env_path)
+
+    if not os.environ.get('GYP_GENERATORS'):
+      # Default to ninja on all platforms.
+      os.environ['GYP_GENERATORS'] = 'ninja'
+
     vs_toolchain.SetEnvironmentAndGetRuntimeDllDirs()