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 d0af148fad74c82c5396c4aadde60099e63f91dc..0c2c4069baabd51dcffbf716e2e645ca0bcf5392 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_;