Revert of Sketch DM refactor.

dm/README

+DM (Diamond Master, a.k.a Dungeon master, a.k.a GM 2).
+
+DM is like GM, but multithreaded.  It doesn't do everything GM does.
+
+DM's design is based around Tasks and a TaskRunner.
+
+A Task represents an independent unit of work that might fail.  We make a task
+for each GM/configuration pair we want to run.  Tasks can kick off new tasks
+themselves.  For example, a CpuTask can kick off a ReplayTask to make sure
+recording and playing back an SkPicture gives the same result as direct
+rendering.
+
+The TaskRunner runs all tasks on one of two threadpools, whose sizes are
+configurable by --cpuThreads and --gpuThreads.  Ideally we'd run these on a
+single threadpool but it can swamp the GPU if we shove too much work into it at
+once.  --cpuThreads defaults to the number of cores on the machine.
+--gpuThreads defaults to 1, but you may find 2 or 4 runs a little faster.
+
+So the main flow of DM is:
+
+    for each GM:
+        for each configuration:
+            kick off a new task
+    < tasks run, maybe fail, and maybe kick off new tasks >
+    wait for all tasks to finish
+    report failures
+