Reviewed by Mihai Parparita.

        Add more unit tests for rebaseline-chromium-webkit-tests. This
        change involves restructuring a bunch of r-c-w-t code to make it
        more testable as well. We also add wrapper classes for handling
        testing zip files and fetching URLs.

        https://bugs.webkit.org/show_bug.cgi?id=53040

        * Scripts/webkitpy/common/system/urlfetcher.py:
        * Scripts/webkitpy/common/system/urlfetcher_mock.py:
        * Scripts/webkitpy/common/system/zipfileset_mock.py:
        * Scripts/webkitpy/layout_tests/rebaseline_chromium_webkit_tests.py:
        * Scripts/webkitpy/layout_tests/rebaseline_chromium_webkit_tests_unittest.py:
        * Scripts/webkitpy/tool/mocktool.py:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@77095 268f45cc-cd09-0410-ab3c-d52691b4dbfc