Don't require -DSK_USE_POSIX_THREADS.

gyp/common_conditions.gypi

 # conditions used in both common.gypi and skia.gyp in chromium
 #
 {
   'defines': [
     'SK_ALLOW_STATIC_GLOBAL_INITIALIZERS=<(skia_static_initializers)',
     'SK_SUPPORT_GPU=<(skia_gpu)',
     'SK_SUPPORT_OPENCL=<(skia_opencl)',
     'SK_FORCE_DISTANCE_FIELD_TEXT=<(skia_force_distance_field_text)',
   ],
   'conditions' : [
@@ skipping 614 matching lines @@
               'SK_SUPPORT_LEGACY_CLIPTOLAYERFLAG',
             ],
           }],
           [ 'skia_profile_enabled == 1', {
             'cflags': ['-g', '-fno-omit-frame-pointer', '-marm', '-mapcs'],
           }],
         ],
       },
     ],
 
-    # We can POD-style initialization of static mutexes to avoid generating
-    # static initializers if we're using a pthread-compatible thread interface.
-    [ 'skia_os != "win"', {
-      'defines': [
-        'SK_USE_POSIX_THREADS',
-      ],
-    }],
-
     [ 'skia_moz2d', {
       'defines': [
         # add flags here (e.g. SK_SUPPORT_LEGACY_...) needed by moz2d
       ],
     }],
 
     [ 'skia_is_bot', {
       'defines': [ 'SK_CRASH_HANDLER' ],
     }],
 
   ], # end 'conditions'
   # The Xcode SYMROOT must be at the root. See build/common.gypi in chromium for more details
   'xcode_settings': {
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }