Reviewed by David Levin.

        webkit-patch should support CHANGE_LOG_EDIT_APPLICATION
        https://bugs.webkit.org/show_bug.cgi?id=39546

        One sublty is that we want to wait for the user to finish editing the
        ChangeLog before moving on to the next step.  That means we want to pass
        -W to open.  However, if the user is using Xcode to edit the ChangeLog,
        we don't want them to have to exit the Xcode application.  For this reason,
        we create a new instance of the application with -n.

        Overall, xed seems like a better solution, so we recommend that too.

        * Scripts/webkitpy/common/system/user.py:
        * Scripts/webkitpy/tool/mocktool.py:
        * Scripts/webkitpy/tool/steps/editchangelog.py:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@60384 268f45cc-cd09-0410-ab3c-d52691b4dbfc