Check if the GN args file exists before access.

Otherwise I get the following error running "mojob.py build":

Building in out/Debug ...
Traceback (most recent call last):
  File "mojo/tools/mojob.py", line 267, in <module>
    sys.exit(main())
  File "mojo/tools/mojob.py", line 263, in main
    return args.func(args)
  File "mojo/tools/mojob.py", line 87, in build
    if get_gn_arg_value(out_dir, 'use_goma') == 'true':
  File "mojo/tools/mojob.py", line 76, in get_gn_arg_value
    with open(os.path.join(out_dir, "args.gn"), "r") as args_file:
IOError: [Errno 2] No such file or directory: 'out/Debug/args.gn'

BUG=NONE
TEST="mojob.py build" works without an args.gn file.
R=jam@chromium.org

Review URL: https://codereview.chromium.org/723713002