Move OAuth2 client id and secret switches to gaia_switches.h

The OAuth2 client id and secret switches are also used on Chrome for iOS
downstream so they need to be available in a public header.

BUG=285944

Review URL: https://codereview.chromium.org/141653020

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250419 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250419