Handle tts utility process teardown on component extension unload

This change handles a corner case where the tts utility process stays around
after the component extension gets unloaded. On re-load, the utility process is
re-started correctly without this change (i.e. there are not multiple tts utility processes run), so
this change is a minor cleanup.

R=dmazzoni@chromium.org

Test: trigger tts (e.g. via ChromeVox), attach to the utility process and wait
for the tts component extension to be evicted after 15 seconds. Verify the
utility process exits cleanly.

Change-Id: Ieadf44d45ba902e298bf4497bc7433d44aa1a6cf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2471803Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817194}

chromeos/services/tts/tts_service.cc

@@ -61,6 +61,11 @@ void TtsService::BindTtsStream(
     mojo::PendingReceiver<mojom::TtsStream> receiver,
     mojo::PendingRemote<audio::mojom::StreamFactory> factory) {
   stream_receiver_.Bind(std::move(receiver));
+  stream_receiver_.set_disconnect_handler(base::BindOnce([] {
+    // The remote which lives in component extension js has been disconnected
+    // due to destruction or error.
+    exit(0);
+  }));
 
   // TODO(accessibility): The sample rate below can change based on the audio
   // data retrieved. Plumb this data through and re-create the output device if