chrome.gcm: Add custom permission so that other apps can't snoop our messages
chrome.gcm: Add custom permission so that other apps can't snoop our messages

This technique is specified by the GCM guide:
https://developer.android.com/google/gcm/client.html

MobileChromeApps/cordova-plugin-chrome-apps-gcm,MobileChromeApps/cordova-plugin-chrome-apps-gcm