Simplify the user agent overriding code, in preparation for moving it out of src/webkit.

These workarounds were added a long time ago before Chrome launched. The Microsoft one isn't needed as the download page has a link to the Mac version without the spoof. The Yahoo Japan download page says it's going away in June and just redirects to Microsoft. The gyao.yahoo.co.jp host doesn't use Silverlight now. promotion.shopping.yahoo.co.jp is dead.

BUG=338338
R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/178263007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255534 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255534