Reviewed by Tony Chang.

        new-run-webkit-tests: add a --build-directory command line argument

        Official Google Chrome builds use a non-standard build directory
        location. This patch adds a --build-directory argument that
        allows that location to be specified on the command line. Only
        Chromium-based builds will use this flag for now, but anyone can
        in the future.

        There are no unit tests for this since it's difficult to test
        until mock filesystems are fully supported in the code. This was
        tested by hand for now.

        https://bugs.webkit.org/show_bug.cgi?id=52694

        * Scripts/webkitpy/layout_tests/port/chromium_linux.py:
        * Scripts/webkitpy/layout_tests/port/chromium_mac.py:
        * Scripts/webkitpy/layout_tests/port/chromium_win.py:
        * Scripts/webkitpy/layout_tests/run_webkit_tests.py:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@76101 268f45cc-cd09-0410-ab3c-d52691b4dbfc