Modify ALLOW_UNUSED to allow enabling unused local warnings on MSVC.

mojo/edk/system/data_pipe.cc

Index: mojo/edk/system/data_pipe.cc
diff --git a/mojo/edk/system/data_pipe.cc b/mojo/edk/system/data_pipe.cc
index ace5fd352fe571986dd5db10efb49b70d9b703af..39ad44ac0869658e36a9b3c7792fbbfeccba09f9 100644
--- a/mojo/edk/system/data_pipe.cc
+++ b/mojo/edk/system/data_pipe.cc
@@ -442,7 +442,7 @@ DataPipe::DataPipe(bool has_local_producer,
       producer_two_phase_max_num_bytes_written_(0),
       consumer_two_phase_max_num_bytes_read_(0) {
   // Check that the passed in options actually are validated.
-  MojoCreateDataPipeOptions unused ALLOW_UNUSED = {0};
+  MojoCreateDataPipeOptions unused = {0};
   DCHECK_EQ(ValidateCreateOptions(MakeUserPointer(&validated_options), &unused),
             MOJO_RESULT_OK);
 }