@jason--Could you please file feedback from Cast when you encounter this problem with the Media Router flag turned ON?  To do so, right-click on the Cast icon in the toolbar and choose "Report an Issue".  Put in something unique in the issue description, then reply here with what you entered so that we can locate your feedback report quickly.  Thanks!

Sorry for the delay.  I'll do this first chance I get this week.

Tried following your advice, however, when clicking on the "Report an Issue" option, nothing happened (this was with the media-router flag disabled).  I enabled the media-router flag, restarted chromium, and right clicked on the cast icon again.  This time, there was no "Report an Issue" option, but there was an option to leave feedback.  I didn't leave any feedback though, since the flag was disabled and this didn't seem like the same thing you were talking about.
So... I removed the cast extension and restarted chromium again. This time, I right clicked on the web page I had open and selected "Cast..."  I got the same behaviour as before, "Missing Device".  I disabled the media-router flag, restarted chromium and then the "Cast..." option was missing from the right-click menu... I'm guessing it's tied to the media-router flag.

In the end, to be clear, my original issue was with the Cast EXTENSION.  At the time, I didn't realize the cast feature had made it into the browser natively.  That being said, even after removing the extension, I'm still unable to cast.  At least with the extension installed and the media-router flag disabled, I can still cast.  I realize this doesn't give many technical details, but I hope it is helpful.  Is there anything else I can do to get you any info you need?

So if I understand correctly, this is occurring on Chromium. The reason why it happens is because a part of Media Router (the Media Router extension, which contains the device discovery logic as well as the feedback page) is close sourced, and we don't include them with Chromium by default. Missing MR extension on your Chromium build explains the behavior you are seeing. As of crrev.com/428493, you will be able to pass in --load-media-router-component-extension to load the MR extension on Chromium. Could you give that a try?

Thank you for providing more feedback. Adding requester "skonig@chromium.org" for another review and adding "Needs-Review" label for tracking.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

The media router flag is removed from the source code already as of 56.0.2904.0, see  issue 651255 .

Per Comment 6, this should work correctly in Chromium if you start with the "--load-media-router-component-extension" flag.  If not please re-open.

For completeness:

--show-component-extension-options --load-media-router-component-extension=0 --load-extension=/path/to/media-router-extension

You may or may not need to add --no-proxy-server, depending on your networking setup.