Ensure variables are decoded to unicode.

Ensure variables are decoded to unicode.

Add asserts to ensure variables are unicode. Otherwise it could get into a
situation where variables are left as str and this would cause an assertion.

R=csharp@chromium.org
BUG=

Committed: https://code.google.com/p/swarming/source/detail?repo=client&r=9cc42c3

[M-A, 2013-12-11 14:28:10]

Failure looked like;
'/b/build/slave/android/build/src/out/Debug'
Traceback (most recent call last):
  File "/b/build/slave/android/build/src/tools/swarming_client/isolate.py", line
2181, in main
    return dispatcher.execute(OptionParserIsolate(version=__version__), argv)
  File
"/b/build/slave/android/build/src/tools/swarming_client/third_party/depot_tools/subcommand.py",
line 242, in execute
    return command(parser, args[1:])
  File "/b/build/slave/android/build/src/tools/swarming_client/isolate.py", line
1825, in CMDremap
    complete_state = load_complete_state(options, os.getcwd(), None, False)
  File "/b/build/slave/android/build/src/tools/swarming_client/isolate.py", line
1569, in load_complete_state
    options.extra_variables, options.ignore_broken_items)
  File "/b/build/slave/android/build/src/tools/swarming_client/isolate.py", line
1361, in load_isolate
    cwd, path_variables, relative_base_dir)
  File "/b/build/slave/android/build/src/tools/swarming_client/isolate.py", line
214, in normalize_path_variables
    for k, v in path_variables.iteritems())
  File "/b/build/slave/android/build/src/tools/swarming_client/isolate.py", line
214, in <genexpr>
    for k, v in path_variables.iteritems())
  File "/b/build/slave/android/build/src/tools/swarming_client/isolate.py", line
191, in _normalize_path_variable
    normalized = file_path.get_native_path_case(os.path.normpath(x))
  File
"/b/build/slave/android/build/src/tools/swarming_client/utils/file_path.py",
line 316, in get_native_path_case
    assert isinstance(path, unicode), repr(path)
AssertionError: '/b/build/slave/android/build/src/out/Debug'

so the path was a str instead of a unicode. Upgrading all the variables should
fix this, and this makes sense anyway.

[csharp, 2013-12-11 14:33:57]

lgtm

[M-A, 2013-12-11 14:36:35]

Committed patchset #1 manually as r9cc42c3 (presubmit successful).