DM: --gpu and --cpu should only control top-level tasks.

DM: --gpu and --cpu should only control top-level tasks.

This fixes a bug where we run some Android bots with --nocpu, and the
current behavior disables the (CPU-bound) WriteTasks the GPU bound GM
runs spawn off.  The WriteTasks don't run and we end up with "null" in
our .json files.

Tested locally: out/Release/dm --nocpu -w /tmp/out; ls /tmp/out
  dm.json  gpu/


BUG=skia:2938
Committed: https://skia.googlesource.com/skia/+/53e0be6d2b76321c2012085a73f1e7b269406c78

[mtklein, 2014-09-17 19:15:27]

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

[mtklein, 2014-09-17 19:17:18]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2014-09-17 19:17:47]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/578033002/1

[commit-bot: I haz the power, 2014-09-17 19:17:48]

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 2014-09-18 01:17 UTC

[jcgregorio, 2014-09-17 19:18:54]

On 2014/09/17 19:17:48, I haz the power (commit-bot) wrote:
> 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 2014-09-18 01:17 UTC

LGTM

[mtklein, 2014-09-17 19:21:36]

The CQ bit was unchecked by mtklein@google.com

[mtklein, 2014-09-17 19:21:39]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2014-09-17 19:21:47]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/578033002/1

[commit-bot: I haz the power, 2014-09-17 19:26:22]

Committed patchset #1 (id:1) as 53e0be6d2b76321c2012085a73f1e7b269406c78