[mojo-edk] Expose notification source to MojoWatch callbacks

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 9f6a72e177f4f749890f3d197f1b6dd79f06c181..d8d8abf1791c0e1abefa8292c7bd4feaecbf3814 100644
--- a/mojo/edk/system/data_pipe_producer_dispatcher.cc
+++ b/mojo/edk/system/data_pipe_producer_dispatcher.cc
@@ -473,9 +473,8 @@ void DataPipeProducerDispatcher::NotifyWrite(uint32_t num_bytes) {
 }
 
 void DataPipeProducerDispatcher::OnPortStatusChanged() {
-  // This has to be outside |lock_| because the watch callback can call data
-  // pipe functions which then try to acquire |lock_|.
-  RequestContext request_context;
+  DCHECK(RequestContext::current());
+
   base::AutoLock lock(lock_);
 
   // We stop observing the control port as soon it's transferred, but this can