Revert of SkThreadPool ~~> SkTaskGroup

gyp/dm.gypi

 # GYP for "dm" (Diamond Master, a.k.a Dungeon master, a.k.a GM 2).
 {
   'include_dirs': [
     '../dm',
     '../gm',
     '../tests',
     '../src/images',
     '../src/lazy',
     '../src/core',
     '../src/effects',
@@ skipping 29 matching lines @@
     '../dm/DMSKPTask.cpp',
     '../dm/DMSerializeTask.cpp',
     '../dm/DMTask.cpp',
     '../dm/DMTaskRunner.cpp',
     '../dm/DMTestTask.cpp',
     '../dm/DMUtil.cpp',
     '../dm/DMWriteTask.cpp',
     '../gm/gm.cpp',
     '../gm/gm_expectations.cpp',
 
-    '../src/utils/SkTaskGroup.cpp',
-
     '../src/pipe/utils/SamplePipeControllers.cpp',
     '../src/utils/debugger/SkDebugCanvas.cpp',
     '../src/utils/debugger/SkDrawCommand.cpp',
     '../src/utils/debugger/SkObjectParser.cpp',
     '../tools/LazyDecodeBitmap.cpp',
   ],
   'conditions': [
     [ 'skia_gpu == 1', {
       'dependencies': [ 'gputest.gyp:skgputest' ],
     }],
   ],
 }