Let MojoSetDataPipeConsumerOptions() take null options.

mojo/edk/system/core_unittest.cc

Index: mojo/edk/system/core_unittest.cc
diff --git a/mojo/edk/system/core_unittest.cc b/mojo/edk/system/core_unittest.cc
index 0fdd5acaa62ed013a83295a3819fb88d35b5bcc5..bb8967a08f2567334803b5aa5c6bbfda9dd4fbff 100644
--- a/mojo/edk/system/core_unittest.cc
+++ b/mojo/edk/system/core_unittest.cc
@@ -1250,11 +1250,9 @@ TEST_F(CoreTest, DataPipeSetGetConsumerOptions) {
   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));