DM: make GPU tasks multithreaded again. Big refactor.

dm/DMPipeTask.cpp

 #include "DMPipeTask.h"
 #include "DMUtil.h"
 #include "DMWriteTask.h"
 
 #include "SamplePipeControllers.h"
 #include "SkCommandLineFlags.h"
 #include "SkGPipe.h"
 
 DEFINE_bool(pipe, true, "If true, check several pipe variants against the reference bitmap.");
 
@@ skipping 20 matching lines @@
     } else {
         return "pipe";
     }
 }
 
 PipeTask::PipeTask(const Task& parent,
                    skiagm::GM* gm,
                    SkBitmap reference,
                    bool crossProcess,
                    bool sharedAddressSpace)
-    : Task(parent)
+    : CpuTask(parent)
     , fFlags(get_flags(crossProcess, sharedAddressSpace))
     , fName(UnderJoin(parent.name().c_str(), get_name(fFlags)))
     , fGM(gm)
     , fReference(reference)
     {}
 
 void PipeTask::draw() {
     SkBitmap bitmap;
     SetupBitmap(fReference.colorType(), fGM.get(), &bitmap);
 
@@ skipping 23 matching lines @@
         return true;
     }
     if (fFlags == SkGPipeWriter::kCrossProcess_Flag &&
         fGM->getFlags() & skiagm::GM::kSkipPipeCrossProcess_Flag) {
         return true;
     }
     return false;
 }
 
 }  // namespace DM