Re: [manifest] Clarify display-mode relationship to iframes and legacy content (#309)

@kenchris can you please give a summary of what's come up on blink-dev? 

Options I can see:

 * legacy content doesn't see `display-mode` media feature. 
 * legacy content does see `display-mode` media feature, and it just defaults to browser. 
 * iframes (including cross-origin) just inherit from the above.
  

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https://github.com/w3c/manifest/issues/309#issuecomment-71965657