Let MojoSetDataPipeConsumerOptions() take null options.

The header defining it says that doing so means reset back to the
default options. I forgot about this when implementing it. Oops. :-/

R=vardhan@google.com
BUG=#442

Review URL: https://codereview.chromium.org/1865663002 .
diff --git a/mojo/edk/system/core_unittest.cc b/mojo/edk/system/core_unittest.cc
index 0fdd5ac..bb8967a 100644
--- a/mojo/edk/system/core_unittest.cc
+++ b/mojo/edk/system/core_unittest.cc
@@ -1250,11 +1250,9 @@
   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));