When --mpd is true, run _mpd variants first, then non-mpd.

This makes it easier to benchmark _mpd variants in a profiler.

E.g.,

<profiler> out/Release/nanobench --images --config 8888 --loops -1 --match sp_desk_nytimes

BUG=skia:

Review URL: https://codereview.chromium.org/1184673006