Mojo: Ensure a RequestContext exists on async Channel error processing

If a channel error is triggered from off-thread and posted to the IPC
thread, we weren't ensuring that a RequestContext was in TLS. If the
error lead to event notifications this would cause a nullptr deref.

Fixes that.

BUG=752137
TBR=jcivelli@chromium.org

Change-Id: Iae167a712c9b7d8d853fe805c06afef36d2c4572
Reviewed-on: https://chromium-review.googlesource.com/600291Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491758}

mojo/edk/system/node_controller.cc

@@ -1236,6 +1236,7 @@ void NodeController::OnAcceptPeer(const ports::NodeName& from_node,
 void NodeController::OnChannelError(const ports::NodeName& from_node,
                                     NodeChannel* channel) {
   if (io_task_runner_->RunsTasksInCurrentSequence()) {
+    RequestContext request_context(RequestContext::Source::SYSTEM);
     DropPeer(from_node, channel);
   } else {
     io_task_runner_->PostTask(