We attempt to decrypt incoming messages in the GCM Driver when they're encrypted. This can fail for a variety of reasons, usually by the developer's doing, e.g. invalid header syntax.

There is one case which is on us: not having the right subscription keys available in the database for the received message. This is currently causing ~0.5% of received, encrypted messages to be dropped.

https://cs.chromium.org/chromium/src/components/gcm_driver/gcm_driver.cc?sq=package:chromium&l=303 (the NO_KEYS case)

We should:
  (a) Understand why this has been increasing since October.
  (b) Nuke the affected encryption, since it's not going to do anything anymore.