nacl_io: Add SOCK_DGRAM support to socketpair()

native_client_sdk/src/libraries/nacl_io/fifo_char.h

Index: native_client_sdk/src/libraries/nacl_io/fifo_char.h
diff --git a/native_client_sdk/src/libraries/nacl_io/fifo_char.h b/native_client_sdk/src/libraries/nacl_io/fifo_char.h
index 27cec2f352988c3cdac3aa6efdf95ffff7c3c073..8f026f29e1031d446e57057662884bb6c3bc002f 100644
--- a/native_client_sdk/src/libraries/nacl_io/fifo_char.h
+++ b/native_client_sdk/src/libraries/nacl_io/fifo_char.h
@@ -34,11 +34,11 @@ class FIFOChar : public FIFOInterface {
 
   // Reads out the data making room in the FIFO.  Returns actual amount
   // read.
-  size_t Read(void* buf, size_t len);
+  virtual size_t Read(void* buf, size_t len);
 
   // Writes into the FIFO no more than len bytes, returns actual amount
   // written.
-  size_t Write(const void* buf, size_t len);
+  virtual size_t Write(const void* buf, size_t len);
 
  private:
   char* buffer_;