Release render view's remote objects when it's deleted.

Privously we release them when the window is unloaded, which is not
correct since a render view can have multiple windows (or js contexts)
and when the unload event is emitted the render view could already have
gone.

This PR does the cleaning work purely in browser, so here is no need to
worry about renderer's life time.

browser/api/atom_api_window.cc

@@ -76,6 +76,10 @@ void Window::OnRendererResponsive() {
   Emit("responsive");
 }
 
+void Window::OnRenderViewDeleted() {
+  Emit("render-view-deleted");
+}
+
 void Window::OnRendererCrashed() {
   Emit("crashed");
 }