[mojo-edk] Expose notification source to MojoWatch callbacks

mojo/edk/system/node_channel.cc

Index: mojo/edk/system/node_channel.cc
diff --git a/mojo/edk/system/node_channel.cc b/mojo/edk/system/node_channel.cc
index 0e2ef52769603ffe4fbd63debf02e68bf257a3e1..4072ea086a10b1e8e406f4b15f328c8712c52c1e 100644
--- a/mojo/edk/system/node_channel.cc
+++ b/mojo/edk/system/node_channel.cc
@@ -10,6 +10,7 @@
 
 #include "base/logging.h"
 #include "mojo/edk/system/channel.h"
+#include "mojo/edk/system/request_context.h"
 
 namespace mojo {
 namespace edk {
@@ -347,6 +348,8 @@ void NodeChannel::OnChannelMessage(const void* payload,
                                    ScopedPlatformHandleVectorPtr handles) {
   DCHECK(io_task_runner_->RunsTasksOnCurrentThread());
 
+  RequestContext request_context(RequestContext::Source::SYSTEM);
+
 #if defined(OS_WIN)
   // If we receive handles from a known process, rewrite them to our own
   // process. This can occur when a privileged node receives handles directly
@@ -518,6 +521,8 @@ void NodeChannel::OnChannelMessage(const void* payload,
 void NodeChannel::OnChannelError() {
   DCHECK(io_task_runner_->RunsTasksOnCurrentThread());
 
+  RequestContext request_context(RequestContext::Source::SYSTEM);
+
   ShutDown();
   // |OnChannelError()| may cause |this| to be destroyed, but still need access
   // to the name name after that destruction. So may a copy of