We meant these to be YES... sigh.

gyp/common_conditions.gypi

 # Copyright 2015 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # conditions used in both common.gypi and skia.gyp in chromium
 #
 {
   'defines': [
     'SK_ALLOW_STATIC_GLOBAL_INITIALIZERS=<(skia_static_initializers)',
@@ skipping 524 matching lines @@
               # The -L/usr/lib is redundant but innocuous: it's a default path.
               'LIBRARY_SEARCH_PATHS': [ '/usr/lib -fsanitize=<(skia_sanitizer)'],
             }],
           ],
           'CLANG_CXX_LIBRARY':                         'libc++',
           'CLANG_CXX_LANGUAGE_STANDARD':               'c++11',
           'GCC_ENABLE_CPP_EXCEPTIONS':                 'NO',   # -fno-exceptions
           'GCC_ENABLE_CPP_RTTI':                       'NO',   # -fno-rtti
           'GCC_THREADSAFE_STATICS':                    'NO',   # -fno-threadsafe-statics
           'GCC_ENABLE_SUPPLEMENTAL_SSE3_INSTRUCTIONS': 'YES',  # -mssse3
-          'GCC_SYMBOLS_PRIVATE_EXTERN':                'NO',   # -fvisibility=hidden
-          'GCC_INLINES_ARE_PRIVATE_EXTERN':            'NO',   # -fvisibility-inlines-hidden
+          'GCC_SYMBOLS_PRIVATE_EXTERN':                'YES',  # -fvisibility=hidden
+          'GCC_INLINES_ARE_PRIVATE_EXTERN':            'YES',  # -fvisibility-inlines-hidden
           'GCC_CW_ASM_SYNTAX':                         'NO',   # remove -fasm-blocks
           'GCC_ENABLE_PASCAL_STRINGS':                 'NO',   # remove -mpascal-strings
           'WARNING_CFLAGS': [
             '-Wall',
             '-Wextra',
             '-Winit-self',
             '-Wpointer-arith',
             '-Wsign-compare',
 
             '-Wno-unused-parameter',
@@ skipping 34 matching lines @@
           'CODE_SIGNING_REQUIRED': 'NO',
           'IPHONEOS_DEPLOYMENT_TARGET': '<(ios_sdk_version)',
           'SDKROOT': 'iphoneos',
           'TARGETED_DEVICE_FAMILY': '1,2',
 
           'CLANG_CXX_LIBRARY':              'libc++',
           'CLANG_CXX_LANGUAGE_STANDARD':    'c++11',
           'GCC_ENABLE_CPP_EXCEPTIONS':      'NO',   # -fno-exceptions
           'GCC_ENABLE_CPP_RTTI':            'NO',   # -fno-rtti
           'GCC_THREADSAFE_STATICS':         'NO',   # -fno-threadsafe-statics
-          'GCC_SYMBOLS_PRIVATE_EXTERN':     'NO',   # -fvisibility=hidden
-          'GCC_INLINES_ARE_PRIVATE_EXTERN': 'NO',   # -fvisibility-inlines-hidden
+          'GCC_SYMBOLS_PRIVATE_EXTERN':     'YES',  # -fvisibility=hidden
+          'GCC_INLINES_ARE_PRIVATE_EXTERN': 'YES',  # -fvisibility-inlines-hidden
 
           'GCC_THUMB_SUPPORT': 'NO',  # TODO(mtklein): why would we not want thumb?
         },
       },
     ],
 
     [ 'skia_os == "android"',
       {
         'defines': [
           'SK_BUILD_FOR_ANDROID',
@@ skipping 73 matching lines @@
           'LD_RUNPATH_SEARCH_PATHS': ['@executable_path/.'],
       },
     }],
 
   ], # end 'conditions'
   # The Xcode SYMROOT must be at the root. See build/common.gypi in chromium for more details
   'xcode_settings': {
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }