Delete TODO-comment about guest process/frame IDs in MessageService::OpenChannelImpl

Deleted a comment in MessageService::OpenChannelImpl that
questioned the use of |guest_process_id| and
|guest_render_frame_routing_id|. These ID appear to be used by some
component extensions (bug 424762) so they cannot be removed.

Review-Url: https://codereview.chromium.org/2529213002
Cr-Commit-Position: refs/heads/master@{#439504}

chrome/browser/extensions/api/messaging/message_service.cc

@@ -566,11 +566,6 @@ void MessageService::OpenChannelImpl(BrowserContext* browser_context,
   channel->receiver.reset(params->receiver.release());
   AddChannel(std::move(channel_ptr), params->receiver_port_id);
 
-  // TODO(robwu): Could |guest_process_id| and |guest_render_frame_routing_id|
-  // be removed? In the past extension message routing was process-based, but
-  // now that extensions are routed from a specific RFH, the special casing for
-  // guest views seems no longer necessary, because the ExtensionMessagePort can
-  // simply obtain the source process & frame ID directly from the RFH.
   int guest_process_id = content::ChildProcessHost::kInvalidUniqueID;
   int guest_render_frame_routing_id = MSG_ROUTING_NONE;
   if (params->include_guest_process_info) {