Remove _unit_test_config hack, instead use FileSystem() in cpp.py & cpp_unittest.py

Tools/Scripts/webkitpy/style/checkers/cpp*.py scripts use _unit_test_config global variable as a scaffholding, to imitate opening of mock header {contents}. It is a hack and should be avoided. Instead we should use filesystem (FileSystem()) object for the purpose.

BUG=305507

TEST=None; no behavior change;

Review URL: https://codereview.chromium.org/32663002

git-svn-id: svn://svn.chromium.org/blink/trunk@160363 bbb929c8-8fbe-4397-9dbb-9b2b20218538