Fix test harness to correctly split extra flags.

tools/testrunner/local/execution.py

 # Copyright 2012 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 #       copyright notice, this list of conditions and the following
 #       disclaimer in the documentation and/or other materials provided
@@ skipping 153 matching lines @@
     d8testflag = []
     shell = test.suite.shell()
     if shell == "d8":
       d8testflag = ["--test"]
     if utils.IsWindows():
       shell += ".exe"
     cmd = (self.context.command_prefix +
            [os.path.abspath(os.path.join(self.context.shell_dir, shell))] +
            d8testflag +
            test.suite.GetFlagsForTestCase(test, self.context) +
-           [self.context.extra_flags])
+           self.context.extra_flags)
     return cmd
 
 
 class BreakNowException(Exception):
   def __init__(self, value):
     self.value = value
   def __str__(self):
     return repr(self.value)