Let MojoSetDataPipeConsumerOptions() take null options.

mojo/edk/system/core_unittest.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "mojo/edk/system/core.h"
 
 #include <stdint.h>
 
 #include <limits>
 
@@ skipping 1232 matching lines @@
   copts.struct_size = kCoptsSize;
   copts.read_threshold_num_bytes = 16;
   EXPECT_EQ(MOJO_RESULT_OK,
             core()->SetDataPipeConsumerOptions(ch, MakeUserPointer(&copts)));
   copts = MojoDataPipeConsumerOptions();
   EXPECT_EQ(MOJO_RESULT_OK, core()->GetDataPipeConsumerOptions(
                                 ch, MakeUserPointer(&copts), kCoptsSize));
   EXPECT_EQ(kCoptsSize, copts.struct_size);
   EXPECT_EQ(16u, copts.read_threshold_num_bytes);
 
-  // Default read threshold.
-  copts.struct_size = kCoptsSize;
-  copts.read_threshold_num_bytes = 0;
+  // Can also set to default by passing null.
   EXPECT_EQ(MOJO_RESULT_OK,
-            core()->SetDataPipeConsumerOptions(ch, MakeUserPointer(&copts)));
+            core()->SetDataPipeConsumerOptions(ch, NullUserPointer()));
   copts = MojoDataPipeConsumerOptions();
   EXPECT_EQ(MOJO_RESULT_OK, core()->GetDataPipeConsumerOptions(
                                 ch, MakeUserPointer(&copts), kCoptsSize));
   EXPECT_EQ(kCoptsSize, copts.struct_size);
   // Note: Should be reported as 0 ("default"), even if it means the element
   // struct_size.
   EXPECT_EQ(0u, copts.read_threshold_num_bytes);
 
   EXPECT_EQ(MOJO_RESULT_OK, core()->Close(ph));
   EXPECT_EQ(MOJO_RESULT_OK, core()->Close(ch));
@@ skipping 277 matching lines @@
   EXPECT_EQ(MOJO_RESULT_BUSY, waiter.result);
 
   EXPECT_EQ(MOJO_RESULT_OK, core()->Close(h));
 }
 
 // TODO(vtl): Test |DuplicateBufferHandle()| and |MapBuffer()|.
 
 }  // namespace
 }  // namespace system
 }  // namespace mojo