Restore a really single-threaded mode to DM.

Used to be:
   0 -> run on main thread plus an autodetected number of extra threads (default)
   N -> run on main thread plus N extra threads

Now it's:
  -1 -> run on main thread plus an autodetected number of extra threads (default)
   0 -> run on main thread
   N -> run on main thread plus N extra threads

BUG=skia:

Review URL: https://codereview.chromium.org/636593002
3 files changed