Make DM play nice with the plan to split CPU and GPU work.

Make DM play nice with the plan to split CPU and GPU work.

--nocpu skips all CPU-bound work.
--nogpu skips all GPU-bound work.

/m/s/skia (dm) $ r dm --nogpu
ninja: Entering directory `out/Release'
[17/17] LINK dm
(294 GMs, 620 benches) x 4 configs, 245 tests
0 tasks left
71.46user 12.40system 0:04.52elapsed 1854%CPU (0avgtext+0avgdata 16828656maxresident)k
0inputs+0outputs (0major+3252811minor)pagefaults 0swaps

/m/s/skia (dm) $ r dm --nocpu
ninja: Entering directory `out/Release'
ninja: no work to do.
(294 GMs, 620 benches) x 4 configs, 245 tests
926 tasks leftUnsupported vertex-color/texture xfer mode.
Unsupported vertex-color/texture xfer mode.
0 tasks left
11.10user 2.38system 0:17.15elapsed 78%CPU (0avgtext+0avgdata 1310896maxresident)k
16inputs+4664outputs (0major+161322minor)pagefaults 0swaps

BUG=skia:2074
R=halcanary@google.com, borenet@google.com, mtklein@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/219403004

git-svn-id: http://skia.googlecode.com/svn/trunk@14001 2bbb7eff-a529-9590-31e7-b0007b416f81