Add a NaCl-Private interface for opening files in DIR_PNACL_COMPONENT.

ppapi/c/private/ppb_nacl_private.h

Index: ppapi/c/private/ppb_nacl_private.h
diff --git a/ppapi/c/private/ppb_nacl_private.h b/ppapi/c/private/ppb_nacl_private.h
index dc7e7a23aed76696a9237cf7f11d02260475d85c..d30b6fed880fa170981d8b5b69cc4bd4b4d572c9 100644
--- a/ppapi/c/private/ppb_nacl_private.h
+++ b/ppapi/c/private/ppb_nacl_private.h
@@ -54,6 +54,12 @@ struct PPB_NaCl_Private {
                                void** target_handle,
                                uint32_t desired_access,
                                uint32_t options);
+
+  // Returns a read-only file descriptor of a file rooted in the Pnacl
+  // component directory, or -1 on error.
+  // Do we want this to take a completion callback and be async, or
+  // or could we make this happen on another thread?
+  int (*GetReadonlyPnaclFd)(const char* filename);
 };
 
 #endif  // PPAPI_C_PRIVATE_PPB_NACL_PRIVATE_H_