Revert of SkThreadPool ~~> SkTaskGroup

dm/DMTaskRunner.cpp

 #include "DMTaskRunner.h"
 #include "DMTask.h"
 
 namespace DM {
 
-void TaskRunner::add(CpuTask* task) { fCpuWork.add(task);  }
-void TaskRunner::add(GpuTask* task) { fGpuWork.push(task); }
+TaskRunner::TaskRunner(int cpuThreads, int gpuThreads) : fCpu(cpuThreads), fGpu(gpuThreads) {}
+
+void TaskRunner::add(CpuTask* task) { fCpu.add(task); }
+void TaskRunner::addNext(CpuTask* task) { fCpu.addNext(task); }
+void TaskRunner::add(GpuTask* task) { fGpu.add(task); }
 
 void TaskRunner::wait() {
-    GrContextFactory factory;
-    for (int i = 0; i < fGpuWork.count(); i++) {
-        fGpuWork[i]->run(&factory);
-    }
-    fCpuWork.wait();
+    // These wait calls block until each threadpool is done.  We don't allow
+    // spawning new child GPU tasks, so we can wait for that first knowing
+    // we'll never try to add to it later.  Same can't be said of the CPU pool:
+    // both CPU and GPU tasks can spawn off new CPU work, so we wait for that last.
+    fGpu.wait();
+    fCpu.wait();
 }
 
 }  // namespace DM