Add link to the Advanced Font Settings extension in Font Settings

When the extension is installed and enabled, the link
points to the extension's options page. Otherwise, the
link points to the extension's page on the Chrome Web
Store.

This implements the design described in crbug.com/272216

BUG=138993,272216

Review URL: https://codereview.chromium.org/33753004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231017 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231017