FWIW, this also affected other GCM consumers, e.g. https://github.com/morinel/gcmpush/issues/45 and https://github.com/rpush/rpush/issues/292.

Android patch: https://codereview.chromium.org/2265543002

I don't think this can affect desktop since google_apis/gcm/protocol/mcs.proto explicitly restricts AppData to string-string key-value pairs.

Does this mean that our AppData message has gotten out of date with MCS (which should be updated), or is a different protocol being used for Android altogether?

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6688926c9db1c186b24b729fe8c8588f747a1a4a

commit 6688926c9db1c186b24b729fe8c8588f747a1a4a
Author: johnme <johnme@chromium.org>
Date: Mon Aug 22 13:15:18 2016

GCM: Handle non-Strings in incoming message Bundle

Google Cloud Messaging recently started including a "google.sent_time"
Long in the incoming message Bundle from Google Play Services. This
patch filters out non-String values to prevent warnings about
ClassCastExceptions.

BUG= 639313 

Review-Url: https://codereview.chromium.org/2265543002
Cr-Commit-Position: refs/heads/master@{#413435}

[modify] https://crrev.com/6688926c9db1c186b24b729fe8c8588f747a1a4a/components/gcm_driver/android/java/src/org/chromium/components/gcm_driver/GCMDriver.java

No, the MCS server proto still seems to match: https://goto.google.com/jxcgr
The long is being added by the Android Google Play Services app.