Extend functionality of MockFileSystem.move()

MockFileSystem.move() function was able to move only files extended its functionality to move directories also.

Review URL: https://codereview.chromium.org/500453002

git-svn-id: svn://svn.chromium.org/blink/trunk@181195 bbb929c8-8fbe-4397-9dbb-9b2b20218538