Add support for sign in with oauth code

A prep CL for the inline sign in flow, as documented here
https://docs.google.com/a/google.com/document/d/1L0gcqZEZ0lWL0PgqO6uPyecuxJdTvzZpps2zRly_wTg/edit#heading=h.5cwdwtkutcuo

BUG=302037

Review URL: https://codereview.chromium.org/25381004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226879 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226879