Update from https://crrev.com/327068

mojo/tools/mopy/gn.py

 # Copyright 2015 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.
 
 """
 GN-related configuration functions, e.g., to produce a Config object from a GN
 args.gn file).
 """
 
 
@@ skipping 47 matching lines @@
 
   gn_args["dcheck_always_on"] = config.dcheck_always_on
 
   gn_args["mojo_use_nacl"] = config.values.get("use_nacl", False)
 
   gn_args["mojo_use_go"] = config.values.get("mojo_use_go", False)
   if gn_args["mojo_use_go"]:
     gn_args["go_build_tool"] = config.values.get("go_build_tool")
 
   if config.target_os == Config.OS_ANDROID:
-    gn_args["os"] = "android"
+    gn_args["target_os"] = "android"
   elif config.target_os == Config.OS_CHROMEOS:
-    gn_args["os"] = "chromeos"
+    gn_args["target_os"] = "chromeos"
     gn_args["use_glib"] = False
     gn_args["use_system_harfbuzz"] = False
   elif config.target_os == Config.OS_LINUX:
     gn_args["use_aura"] = False
     gn_args["use_glib"] = False
     gn_args["use_system_harfbuzz"] = False
 
   gn_args["target_cpu"] = config.target_cpu
 
   extra_args = config.values.get("gn_args")
@@ skipping 26 matching lines @@
   config_args["sanitizer"] = (
       Config.SANITIZER_ASAN if args.get("is_asan") else None)
   config_args["is_clang"] = args.get("is_clang", False)
   config_args["use_goma"] = args.get("use_goma", False)
   if config_args["use_goma"]:
     config_args["goma_dir"] = args.get("goma_dir")
   config_args["use_nacl"] = args.get("mojo_use_nacl", False)
   config_args["mojo_use_go"] = args.get("mojo_use_go", False)
   if config_args["mojo_use_go"]:
     config_args["go_build_tool"] = args.get("go_build_tool")
-  config_args["target_os"] = args.get("os")
+  config_args["target_os"] = args.get("target_os")
   config_args["target_cpu"] = args.get("target_cpu")
   config_args["dcheck_always_on"] = args.get("dcheck_always_on")
   return Config(**config_args)
 
 
 def ParseGNConfig(build_dir):
   """
   Parse the gn config file present in |build_dir|. This function returns a
   dictionary with boolean values as boolean.
   """
   TRANSLATIONS = {
       "true": "True",
       "false": "False",
   }
   gn_file = os.path.join(build_dir, "args.gn")
   values = {}
   with open(gn_file, "r") as f:
     for line in f.readlines():
       line = re.sub("\s*#.*", "", line)
       result = re.match("^\s*(\w+)\s*=\s*(.*)\s*$", line)
       if result:
         key = result.group(1)
         value = result.group(2)
         values[key] = ast.literal_eval(TRANSLATIONS.get(value, value))
   return values