Handle expired platform key certificates.

Platform keys and certificates are retained until they are manually
cleared or the account is removed from the device.  If a certificate
expires, a new key is now generated and certified as a replacement.

BUG=chromium:322683
TEST=unit

Review URL: https://codereview.chromium.org/110883010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240622 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240622