Remove support for "all or none" two-phase data pipe read/write.

mojo/edk/system/data_pipe_producer_dispatcher.cc

Index: mojo/edk/system/data_pipe_producer_dispatcher.cc
diff --git a/mojo/edk/system/data_pipe_producer_dispatcher.cc b/mojo/edk/system/data_pipe_producer_dispatcher.cc
index ddb8b9f90f247d3be5f3e23f099c65174401e515..536dace9c295c2f286159c77dc66e81f52fb3e85 100644
--- a/mojo/edk/system/data_pipe_producer_dispatcher.cc
+++ b/mojo/edk/system/data_pipe_producer_dispatcher.cc
@@ -84,8 +84,12 @@ MojoResult DataPipeProducerDispatcher::BeginWriteDataImplNoLock(
     MojoWriteDataFlags flags) {
   mutex().AssertHeld();
 
-  return data_pipe_->ProducerBeginWriteData(
-      buffer, buffer_num_bytes, (flags & MOJO_WRITE_DATA_FLAG_ALL_OR_NONE));
+  // This flag may not be used in two-phase mode.
+  if ((flags & MOJO_WRITE_DATA_FLAG_ALL_OR_NONE))
+    return MOJO_RESULT_INVALID_ARGUMENT;
+
+  // TODO(vtl): Remove all-or-none support at lower levels.
+  return data_pipe_->ProducerBeginWriteData(buffer, buffer_num_bytes, false);
 }
 
 MojoResult DataPipeProducerDispatcher::EndWriteDataImplNoLock(