Update Chomium's build files to work w/ latest GN binaries.

mojo/nacl/BUILD.gn

 # All toolchains use the same generated code.
 gen_dir = "$root_build_dir/gen/mojo/nacl"
 
 # Only allow the generator to be run by one toolchain.
 if (current_toolchain == default_toolchain) {
   # Generate the code to plumb the Mojo public API into the NaCl sandbox.
   action("mojo_nacl_codegen") {
     script = "generator/generate_nacl_bindings.py"
     args = [
       "-d",
@@ skipping 91 matching lines @@
       "//native_client/build/config/nacl:nacl_base",
       "//native_client/src/untrusted/irt:irt_core_lib",
       "//native_client/src/untrusted/nacl:imc_syscalls",
       ":mojo_nacl_codegen($default_toolchain)",
     ]
   }
 }
 
 group("mojo_nacl") {
   deps = [
-    ":irt_mojo(//native_client/build/toolchain/nacl:irt_${cpu_arch})",
+    ":irt_mojo(//native_client/build/toolchain/nacl:irt_${current_cpu})",
   ]
 }
 
 group("mojo_nacl_tests") {
   testonly = true
   deps = [
     ":monacl_shell",
-    ":monacl_test(//native_client/build/toolchain/nacl:clang_newlib_${cpu_arch})",
+    ":monacl_test(//native_client/build/toolchain/nacl:clang_newlib_${current_cpu})",
   ]
 }