Move threadpool code from include/utils to tools/threadpool.

gyp/tests.gyp

 # GYP file to build unit tests.
 {
   'includes': [
     'apptype_console.gypi',
   ],
   'targets': [
     {
       'target_name': 'tests',
       'type': 'executable',
       'includes': [
         'pathops_unittest.gypi',
         'tests.gypi',
       ],
-      'dependencies': [ 'tools.gyp:crash_handler' ],
+      'dependencies': [
+         'tools.gyp:crash_handler',
+         'tools.gyp:threadpool',
+      ],
       'sources': [
         '../tests/skia_test.cpp',
       ],
       'conditions': [
         [ 'skia_android_framework == 1', {
           'libraries': [
             '-lskia',
           ],
           'libraries!': [
             '-lz',
             '-llog',
           ],
         }],
         [ 'skia_gpu == 1', {
           'include_dirs': [
             '../src/gpu',
           ],
         }],
       ],
     },
   ],
 }