Share the zygote's fopen overrides with nacl_helper.

sandbox/sandbox_linux.gypi

Index: sandbox/sandbox_linux.gypi
diff --git a/sandbox/sandbox_linux.gypi b/sandbox/sandbox_linux.gypi
index b3489ecfa103384937fec5ec3b5ac17f2911cffa..9cfb897226aaa1a216afad14c8906466c5d9b07e 100644
--- a/sandbox/sandbox_linux.gypi
+++ b/sandbox/sandbox_linux.gypi
@@ -86,5 +86,19 @@
         '..',
       ],
     },
+    {
+      'target_name': 'libc_urandom_override',
+      'type': 'static_library',
+      'sources': [
+        'linux/services/libc_urandom_override.cc',
+        'linux/services/libc_urandom_override.h',
+      ],
+      'dependencies': [
+        '../base/base.gyp:base',
+      ],
+      'include_dirs': [
+        '..',
+      ],
+    },
   ],
 }