Restore a really single-threaded mode to DM.

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:

Committed: https://skia.googlesource.com/skia/+/e356c7f0eeb513d0b8cf7bf7f1f47ba4e997861e

[mtklein, 2014-10-06 18:06:43]

mtklein@google.com changed reviewers:
+ caryclark@google.com, mtklein@google.com

[mtklein, 2014-10-06 18:06:49]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2014-10-06 18:07:03]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/636593002/1

[commit-bot: I haz the power, 2014-10-06 18:07:03]

Note for Reviewers:
The CQ is waiting for an approval.
If you believe that the CL is not ready yet, or if you would like to L-G-T-M
with comments then please uncheck the CQ checkbox.

Waiting for LGTM from a valid reviewer till 2014-10-07 00:07 UTC

[mtklein, 2014-10-06 18:23:16]

lgtm

[commit-bot: I haz the power, 2014-10-06 18:24:34]

Committed patchset #1 (id:1) as e356c7f0eeb513d0b8cf7bf7f1f47ba4e997861e