NaCl: create a separate namespace for Mojo handles.

mojo/public/platform/native/BUILD.gn

 # 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.
 
 assert(!defined(is_nacl) || !is_nacl)
 
 import("../../mojo_sdk.gni")
 
+# For internal use only.
+mojo_sdk_source_set("core_system") {
+  sources = [
+    "core_system_api.h",
+    "core_system_thunks.cc",
+    "core_system_thunks.h",
+  ]
+  defines = [ "MOJO_SYSTEM_IMPLEMENTATION" ]
+  mojo_sdk_deps = [ "mojo/public/c/system" ]
+
+  # The GYP target analogous to this one builds this code into a
+  # static library.  When building for Android, both the GYP and GN
+  # builds add --exclude-libs=ALL globally, which means that all
+  # symbols in static libraries are excluded from export.  That's a
+  # problem, as code outside this target needs to be able to call
+  # MojoSetCoreSystemThunks(). Therefore, the GYP target needs to specifiy
+  # that all dependent targets remove that link flag. Since GN uses a
+  # source_set here, this flag change is not needed.
+}
+
 mojo_sdk_source_set("system") {
   sources = [
     "system_thunks.cc",
     "system_thunks.h",
   ]
   defines = [ "MOJO_SYSTEM_IMPLEMENTATION" ]
   mojo_sdk_deps = [ "mojo/public/c/system" ]
 
   # The GYP target analogous to this one builds this code into a
   # static library.  When building for Android, both the GYP and GN
@@ skipping 31 matching lines @@
     "mojo/public/c/gles2:headers",
     "mojo/public/c/environment",
     "mojo/public/c/system",
   ]
 
   if (is_mac) {
     # TODO(GYP): Make it a run-path dependent library.
     # 'DYLIB_INSTALL_NAME_BASE': '@loader_path',
   }
 }