Add AudioStreamRegistry. Move stream counting logic

content/public/test/mock_render_process_host.cc

Index: content/public/test/mock_render_process_host.cc
diff --git a/content/public/test/mock_render_process_host.cc b/content/public/test/mock_render_process_host.cc
index c7ba2ecffe885cae6e11af745db182cf841ca86b..52a2ee70af68047f664cb196c6cfda81c543b538 100644
--- a/content/public/test/mock_render_process_host.cc
+++ b/content/public/test/mock_render_process_host.cc
@@ -143,7 +143,10 @@ int MockRenderProcessHost::VisibleWidgetCount() const {
   return 1;
 }
 
-void MockRenderProcessHost::AudioStateChanged() {}
+void MockRenderProcessHost::AudioStateChanged() {
+  if (!audio_state_changed_callback_.is_null())
+    audio_state_changed_callback_.Run();
+}
 
 bool MockRenderProcessHost::IsForGuestsOnly() const {
   return is_for_guests_only_;