Clarify null IPC listener support for ChannelProxy

Bug: N/A
Change-Id: I5bb8e59f3a4cf8229bb389636f24243e6b98b64d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2393558Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Dominic Farolino <dom@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805035}

ipc/ipc_channel_proxy.cc

@@ -45,9 +45,7 @@ ChannelProxy::Context::Context(
   // need to either:
   // 1) Create the ChannelProxy on a different thread, or
   // 2) Just use Channel
-  // Note, we currently make an exception for a NULL listener. That usage
-  // basically works, but is outside the intent of ChannelProxy. This support
-  // will disappear, so please don't rely on it. See https://crbug.com/364241
+  // We make an exception for NULL listeners.
   DCHECK(!listener ||
          (ipc_task_runner_.get() != default_listener_task_runner_.get()));
 }