Clear the access token when destroying the Sync backend

BUG=302947
R=pavely

Review URL: https://codereview.chromium.org/26789003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228738 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228738