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
R=jcgregorio@google.com, mtklein@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/578033002