DM: add --serialize

dm/README

 DM is like GM, but multithreaded.  It doesn't do everything GM does yet.
 
 Current approximate list of missing features:
   --mismatchPath
   --missingExpectationsPath
   --writePicturePath
 
   --deferred / --pipe
   --rtree
-  --serialize
   --tiledGrid
 
 
 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