Custom user agent for webview: MacOS Chrome agent fixes youtube embed issues

[dfsm]

For a description of the issue, see this page: https://forums.somethingawful.com/showthread.php?threadid=3837546&pagenumber=110#post517653021

Validated this by:

• Testing multiple times between the MacOS Chrome user agent and no user agent. Issue reliably occurred without any user agent, while never occurring with this change.
• Tried the latest iOS Safari user agent, as maybe the lack of ANY user agent could be the issue (and the Chrome user agent may not be what's fixing it.) However, the issue occurred with this agent value.
• Tried the latest iOS Chrome user agent. As maybe it IS due to Chrome being favoured. However, the issue also occurred with this agent value. Seems more of a Mobile vs Desktop issue.

After all this testing, the agent string being submitted here seems to work reliably. Gonna make this a PR so that you can confirm :)

[nolanw]

I can't reproduce this, on my iPad at least. If I start a scroll on a YouTube embed then it seems to gain focus, as the play button background goes from semitransparent black to solid red, but it doesn't play by itself. But if it helps others then sure!

I'd be happy to revert it at some point if stuff works again, assume it's something to do with the iframe contents and YouTube might fix it on their end. I like to have an Awful-specific user agent so the Forums admins have a sense of how much traffic we're causing, but setting the webview's user agent isn't so bad.