Tested this issue on Window 7,Windows 10 Clients with chrome #58.0.3029.110 

These are the steps followed

1. In the server machine, Enabled Google Cast policy
2. In the server machine, in regedit set the "setting Software\Policies\Google\Chrome\EnableMediaRouter" to Zero
3. In the server machine, i ran "gpupdate /force" in terminal
4. In the Windows 10 Client Machine, i ran "gpupdate /force" in terminal and observed Cast option was disabled.
5. In the Windows 7 Client Machine, i ran "gpupdate /force" in terminal and observed Cast option was Enabled.

Attaching the screen-cast for reference

steve.lawrence@ Could you please look into it and let us know your observations.

Deleted: Issue 724983.mp4 2.3 MB

	Issue 724983.mp4 2.3 MB View Download

@Steve: One thing that I notice now is you say, you directly set registry keys and not GPOs. Those only work when the machine is domain joined. Could it be that the Win 10 machine was not joined to a domain at all. In this case policy will only be accepted if coming from a GPO.

@kklauri: In the video you show the value both being enabled in the GPO editor but having different values in the registry. This rather makes me believe the policy was not applied on the machine. Try to change another policy (one that allows you to set a string is better because it is clear that the new value is in) together with those two to verify its value gets updated each time you force the update.  

Hi, yes the Windows 10 machine is not joined to the domain - it is only a test PC as we don't have any production Windows 10 machines yet.  I will add it to the domain and test, but this does seem to be the likely reason.

I verified on a domain joined win 10 machine that setting the policy through GPO works as expected. I think this is rather stemming from the fact that the machine is not domain-joined. The policy can still work in this case but you should not set the registry key directly rather use gpedit.msc to set it as a local policy instead.
Please let me know if the problem is fixed with that for you and I will close this bug.

I haven't had a chance to test this but I will take your word for it.  You can close this thanks.

Ok. Just ping the bug if you still have issues and we'll reopen it.

Thanks!