Chrome Version: 62.0.3194.0
OS: Mac (likely also Windows, Linux, CrOS)

What steps will reproduce the problem?
(1) Obtain a presentation endpoint, either a Chromecast, a Hangout, or the Cast for EDU Chrome app
(2) Navigate to https://googlechrome.github.io/samples/presentation-api/
(3) Click "new PresentationRequest", "start", then select the device from #1
(4) Wait for presentation to start.  Go to chrome://inspect#other and inspect receiver.html
(5) Set a JS breakpoint in the onmessage handler in receiver.js, and send a message from the page in #2

What is the expected result?
JS breakpoint is hit in devtools.

What happens instead?
JS breakpoint is not hit.  All script execution in receiver.html seems to be stopped.