Revert 255858 "Simplify the user agent code some more since afte..."

Revert 255858 "Simplify the user agent code some more since afte..."

There are some browser_tests failures on the buildbots that don't reproduce locally. Reverting while I investigate.

> Simplify the user agent code some more since after r255534 it's not affected by the site's URL.
> 
> Code in content always gets it now from ContentClient::GetUserAgent. In Chrome, there's a GetUserAgent function which returns it.
> 
> BUG=338338
> R=isherman@chromium.org, pauljensen@chromium.org, scottmg@chromium.org, tfarina@chromium.org
> 
> Review URL: https://codereview.chromium.org/191093002

TBR=jam@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=255867