[CrOS MultiDevice] Update FakeMultiplexedChannel.

Now, when a FakeMultiplexedChannel is disconnected, it notifies its
delegate.

Bug: 824568, 752273
Change-Id: I52a5368b374c7633a09a5c154a37f972d7a207c7
Reviewed-on: https://chromium-review.googlesource.com/1072729Reviewed-by: James Hawkins <jhawkins@chromium.org>
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561665}

chromeos/services/secure_channel/fake_multiplexed_channel.cc

@@ -30,6 +30,8 @@ void FakeMultiplexedChannel::SetDisconnected() {
   DCHECK(!is_disconnected_);
   is_disconnecting_ = false;
   is_disconnected_ = true;
+
+  NotifyDisconnected();
 }
 
 bool FakeMultiplexedChannel::IsDisconnecting() const {