NaCl: create a separate namespace for Mojo handles.

mojo/public/platform/native/BUILD.gn

Index: mojo/public/platform/native/BUILD.gn
diff --git a/mojo/public/platform/native/BUILD.gn b/mojo/public/platform/native/BUILD.gn
index 0e099f99e3765e3293a9b6563ca5753fc6511587..7392f7059da1b2c5ce1dec30f295a7256fbdb4a1 100644
--- a/mojo/public/platform/native/BUILD.gn
+++ b/mojo/public/platform/native/BUILD.gn
@@ -6,6 +6,26 @@ 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",