Disable GCM

[pilgrim-brave]

GCM (Google Cloud Messaging) is a deprecated but still supported way for websites to do push messaging. It routes everything through Google servers. It also provides a chrome.gcm API for extensions, which we should also disable.

Google's developer documentation: https://developers.google.com/cloud-messaging/chrome/client

Chromium code is in and around https://cs.chromium.org/chromium/src/google_apis/gcm/engine/

[LaurenWags]

@pilgrim-brave does this require manual QA? if so, can you provide a test plan please? cc @kjozwiak

[pilgrim-brave]

No.