[css-device-adapt] Clarify no content meta viewport <meta name="viewport"> should be ignored or not

[hiikezoe]

On a site in a webcompat report, there are two meta viewport elements like this;
<meta name="viewport" content="width=device-width,initial-scale=1.0,minimum-scale=1.0,maximum-scale=1.0,user-scalable=0">
...
<meta name="viewport">

As far as I know, in cases where multiple meta viewport elements exist, both Chrome and Firefox uses the last modified one, but in this particular case, Chrome ignores the latter meta viewport, whereas Firefox uses the latter one. To me the Chrome's behavior makes sense, so I am going to change Firefox's behavior in bug 1632020.

Note that both Chrome and Firefox use <meta name="viewport" content="">.