Make GN build work on Mac again.

skia/skia_gn_files.gypi

 # This file is read into the GN build.
 
 # Files are relative to third_party/skia.
 {
   # Shared defines for all builds.
   'skia_feature_defines': [
     'GR_GL_CUSTOM_SETUP_HEADER="GrGLConfig_chrome.h"',
     'GR_GL_IGNORE_ES3_MSAA=0',
     'SK_ATTR_DEPRECATED=SK_NOTHING_ARG1',
     'SK_DEFERRED_CANVAS_USES_FACTORIES=1',
@@ skipping 839 matching lines @@
     '<(skia_src_path)/utils/SkThreadUtils_pthread_other.cpp',
     '<(skia_src_path)/utils/SkThreadUtils_win.cpp',
     '<(skia_src_path)/utils/SkThreadUtils_win.h',
     '<(skia_src_path)/utils/SkTFitsIn.h',
     '<(skia_src_path)/utils/SkTLogic.h',
     '<(skia_src_path)/utils/SkUnitMappers.cpp',
 
     #mac
     '<(skia_include_path)/utils/mac/SkCGUtils.h',
     '<(skia_src_path)/utils/mac/SkCreateCGImageRef.cpp',
+    '<(skia_src_path)/utils/mac/SkStream_mac.cpp',
 
     #windows
     '<(skia_include_path)/utils/win/SkAutoCoInitialize.h',
     '<(skia_include_path)/utils/win/SkHRESULT.h',
     '<(skia_include_path)/utils/win/SkIStream.h',
     '<(skia_include_path)/utils/win/SkTScopedComPtr.h',
     '<(skia_src_path)/utils/win/SkAutoCoInitialize.cpp',
     '<(skia_src_path)/utils/win/SkDWrite.h',
     '<(skia_src_path)/utils/win/SkDWrite.cpp',
     '<(skia_src_path)/utils/win/SkDWriteFontFileStream.cpp',
     '<(skia_src_path)/utils/win/SkDWriteFontFileStream.h',
     '<(skia_src_path)/utils/win/SkDWriteGeometrySink.cpp',
     '<(skia_src_path)/utils/win/SkDWriteGeometrySink.h',
     '<(skia_src_path)/utils/win/SkHRESULT.cpp',
     '<(skia_src_path)/utils/win/SkIStream.cpp',
     '<(skia_src_path)/utils/win/SkWGL_win.cpp',
 
     #testing
     '<(skia_src_path)/fonts/SkGScalerContext.cpp',
     '<(skia_src_path)/fonts/SkGScalerContext.h',
   ],
 }