Reviewed by Jeremy Orlow.

        webkitpy: ScriptError('Failed to run "[u\'taskkill.exe\', u\'/f\', u\'/im\', u\'httpd.exe\']" exit_code: 128',)
        https://bugs.webkit.org/show_bug.cgi?id=38248

        The previous code did not check the return code of taskkill.
        When I moved that callsite from using subprocess.call to
        Executive.run_command having a non-zero return code became an error.

        In this change I've centralized our killall handling in executive,
        and added tests for it to make sure it works.

        Currently kill_process and kill_all swallow exceptions in the cases
        where the process(es) to be killed do(es) not exist.

        * Scripts/webkitpy/common/system/executive.py:
        * Scripts/webkitpy/common/system/executive_unittest.py:
        * 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/port/gtk.py:
        * Scripts/webkitpy/layout_tests/port/mac.py:
        * Scripts/webkitpy/layout_tests/port/qt.py:
        * Scripts/webkitpy/layout_tests/port/win.py:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@58397 268f45cc-cd09-0410-ab3c-d52691b4dbfc