In the Google Cloud Messaging for Chrome documentation there are no information about Topic Messaging feature. https://developers.google.com/cloud-messaging/chrome/client
Is not yet available?
Here is the GCM reference docs without subscribe method https://developer.chrome.com/apps/gcm
And here is the Android and iOS reference with the GcmPubSub class that provides subscribe/unsubscribe methods: https://developers.google.com/android/reference/com/google/android/gms/gcm/GcmPubSub https://developers.google.com/cloud-messaging/ios/api/gcm/interface_g_c_m_pub_sub
Unfortunately it is not yet implemented. Please reach out here: https://groups.google.com/forum/#!forum/gcm-for-chrome-feedback to get in touch with developers or file a feature request here: https://code.google.com/p/chromium/issues/list