DCHECK that listener thread != IO thread

ipc/ipc_sync_channel.cc

Index: ipc/ipc_sync_channel.cc
diff --git a/ipc/ipc_sync_channel.cc b/ipc/ipc_sync_channel.cc
index b2e79a86f7363b289330cf53207e1021a801ead6..9e04f61dbca13f740c1fb5e47fea9f26e99aeebd 100644
--- a/ipc/ipc_sync_channel.cc
+++ b/ipc/ipc_sync_channel.cc
@@ -412,6 +412,9 @@ SyncChannel::SyncChannel(
     bool create_pipe_now,
     WaitableEvent* shutdown_event)
     : ChannelProxy(new SyncContext(listener, ipc_task_runner, shutdown_event)) {
+  // The current (listener) thread must be distinct from the IPC thread, or else
+  // sending synchronous messages will deadlock.
+  DCHECK_NE(ipc_task_runner, base::ThreadTaskRunnerHandle::Get());
   ChannelProxy::Init(channel_handle, mode, create_pipe_now);
   StartWatching();
 }
@@ -421,6 +424,9 @@ SyncChannel::SyncChannel(
     base::SingleThreadTaskRunner* ipc_task_runner,
     WaitableEvent* shutdown_event)
     : ChannelProxy(new SyncContext(listener, ipc_task_runner, shutdown_event)) {
+  // The current (listener) thread must be distinct from the IPC thread, or else
+  // sending synchronous messages will deadlock.
+  DCHECK_NE(ipc_task_runner, base::ThreadTaskRunnerHandle::Get());
   StartWatching();
 }