The Chromecast team have indicated they won't fix this bug.
If you are a developer shipping with this library, you can't do anything about it according to Chromecast team. You can only inform users to ignore the errors. (I believe Chromecast team is not entirely correct as the library could, at the least, avoid requesting the extension scipt if the browser is not Chrome. And I suspect it could be possible to suppress the error even if it is Chrome, but haven't tried anything.)
If you are a user annoyed by these console messages, you can switch to Chrome if not using it already.
If you are a user annoyed by these console messages, you can switch to Chrome if not using it already and install the Chromecast extension. (I mention that because people are landing here after searching for this error.)
Update [Nov 13, 2014]: The problem has now been acknowledged by Google. A member of the Chromecast team seems to suggest the issue will be bypassed by a change the team is currently working on.
Update 2 [Feb 17, 2015]: The team claim there's nothing they can do to remove the error logs as it's a standard Chrome network error and they are still working on a long-term fix. Public comments on the bug tracker were closed with that update.
Wow. This is like leaving all your console.log() statements in a production product.
@RenaissanceProgrammer Using Chrome and installing Chromecast solves it for you individually, yes. The question is more aimed at programmers who want to remove the problem for their end-users.
Hey, it's now Feb, '16 and I have the latest version of Chrome and it hasn't been fixed. Any updates?
I hope they fix it soon, cause we're still getting those errors without the Chromecast extension.
It's March 7th 2016. Chrome just updated, I'm still getting the console errors.
After being forced to install the plugin, I get 4 'untrusted origin' warnings from the same script. It's over, Google won.