Export only Cronet API symbols and hide all others.

components/cronet.gypi

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 {
   'conditions': [
     ['OS=="android"', {
       'targets': [
         {
           'target_name': 'cronet_jni_headers',
@@ skipping 755 matching lines @@
           ],
           'dependencies': [
             'cronet_version_header',
             '../base/base.gyp:base',
             '../net/net.gyp:net',
           ],
           'cflags': [
             '-fdata-sections',
             '-ffunction-sections',
             '-fno-rtti',
+            '-fvisibility=hidden'
             '-fvisibility-inlines-hidden',
             '-Wno-sign-promo',
             '-Wno-missing-field-initializers',
           ],
           'ldflags': [
             '-llog',
             '-Wl,--gc-sections',
             '-Wl,--exclude-libs,ALL'
           ],
         },
@@ skipping 99 matching lines @@
                   'cronet/ios/test/cronet_bidirectional_stream_test.mm',
                   'cronet/ios/test/cronet_test_runner.mm',
                 ],
               },
             ],
           },
       ],
     }],  # OS=="ios"
   ],
 }