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

Review URL: https://codereview.chromium.org/192283002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255867 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255867