* Add API function to send a message to a specific frameId in a tab
* Update _windowMessageHandlers to support additional info per handler
* Remove message token
* Add new authorization check
* Set up new initialization handler
* Update initialization
* Remove message token
* Replace 'prepare' with 'configure'
* Create new prepare function
* Change configure guard
* Log errors in onMessage
* Improve popup initialize function
* Clear secret/token in _resetFrame
* Remove backend message token
* Clear src and srcdoc attributes before loading
* Don't treat about:blank unloads as load events
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