Remove webkitpy.thirdparty.mock, webkitpy.common.webkitunittest.

More webkitpy cleanup.

webkitunittest isn't needed once we required Python 2.7.

mock.Mock() was really only being used in one file (and didn't need to be used
there, either), and I'm not generally fan of the generic Mock approach
it uses (preferring concrete mock objects or fakes).

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/467133003

git-svn-id: svn://svn.chromium.org/blink/trunk@180404 bbb929c8-8fbe-4397-9dbb-9b2b20218538