Revert of Turn SkTaskGroups back on.

gyp/tools.gyp

 # GYP file to build various tools.
 #
 # To build on Linux:
 #  ./gyp_skia tools.gyp && make tools
 #
 {
   'includes': [
     'apptype_console.gypi',
   ],
   'targets': [
@@ skipping 142 matching lines @@
         '../tools/skdiff_utils.h',
       ],
       'dependencies': [
         'skia_lib.gyp:skia_lib',
       ],
     },
     {
       'target_name': 'skpdiff',
       'type': 'executable',
       'sources': [
+        '../src/core/SkTaskGroup.cpp',
         '../tools/skpdiff/skpdiff_main.cpp',
         '../tools/skpdiff/SkDiffContext.cpp',
         '../tools/skpdiff/SkImageDiffer.cpp',
         '../tools/skpdiff/SkPMetric.cpp',
         '../tools/skpdiff/skpdiff_util.cpp',
       ],
       'include_dirs': [
         '../src/core/', # needed for SkTLList.h
         '../tools/',    # needed for picture_utils::replace_char
       ],
@@ skipping 533 matching lines @@
               'flags.gyp:flags',
               'skia_lib.gyp:skia_lib',
               'resources',
             ],
           },
         ],
       },
     ],
   ],
 }