When launching PNaCl helper nexes, explicitly disable IRT loading.

ppapi/api/private/ppb_nacl_private.idl

Index: ppapi/api/private/ppb_nacl_private.idl
diff --git a/ppapi/api/private/ppb_nacl_private.idl b/ppapi/api/private/ppb_nacl_private.idl
index aa65b97f18d89efc6ab8ec22c5927d416d716366..11f13e2682ac5af9cf16afdf5ffcd24acaab87fb 100644
--- a/ppapi/api/private/ppb_nacl_private.idl
+++ b/ppapi/api/private/ppb_nacl_private.idl
@@ -44,12 +44,15 @@ interface PPB_NaCl_Private {
    * indicates that the nexe run by sel_ldr will use the PPAPI APIs.
    * This implies that LaunchSelLdr is run from the main thread.  If a nexe
    * does not need PPAPI, then it can run off the main thread.
+   * The |uses_irt| flag indicates if a nexe depends on the IRT or not.

[Mark Seaborn, 2013/01/04 00:05:41]

Maybe "...indicates whether the IRT should be loaded in this NaCl process"?

[jvoung (off chromium), 2013/01/04 17:09:23]

Done.

+   * This is true for ABI stable nexes.
    */
   PP_NaClResult LaunchSelLdr([in] PP_Instance instance,
                              [in] str_t alleged_url,
                              [in] PP_Bool uses_ppapi,
                              [in] PP_Bool enable_ppapi_dev,
                              [in] int32_t socket_count,

[Mark Seaborn, 2013/01/04 00:05:41]

Can you keep socket_count and imc_handles together?  You can put uses_irt next
to the other PP_Bools.

I'd put "uses_irt" earlier in the argument list, before "uses_ppapi" (since
that's only temporary) and before "enable_ppapi_dev" (since it's only a dev
option).

[jvoung (off chromium), 2013/01/04 17:09:23]

Done.  Actually |uses_ppapi| is also toggled to false for PNaCl, so I'm not sure
how temporary that is.  It may hang around until the nacl trusted plugin logic
is rewritten?

+                             [in] PP_Bool uses_irt,
                              [out] mem_t imc_handles);
 
   /* This function starts the IPC proxy so the nexe can communicate with the