Mojo: DataPipe: Implement "may discard" mode for simple writes.

mojo/system/data_pipe.cc

Index: mojo/system/data_pipe.cc
diff --git a/mojo/system/data_pipe.cc b/mojo/system/data_pipe.cc
index d56758ad067709446b1cc8957ff3d9c4b2c9b594..dac187ecc3fe7d4129716a69aa5a4825a4019fcb 100644
--- a/mojo/system/data_pipe.cc
+++ b/mojo/system/data_pipe.cc
@@ -37,6 +37,9 @@ MojoResult DataPipe::ValidateOptions(
     return MOJO_RESULT_INVALID_ARGUMENT;
   out_options->struct_size = static_cast<uint32_t>(sizeof(*out_options));
 
+  // All flags are okay (unrecognized flags will be ignored).
+  out_options->flags = in_options->flags;
+
   if (in_options->element_num_bytes == 0)
     return MOJO_RESULT_INVALID_ARGUMENT;
   out_options->element_num_bytes = in_options->element_num_bytes;