[sync] Infrastructure for auth error integration tests

This CL adds infrastructure for writing sync integration tests for
various scenarios involving OAuth2 token fetches, along with a
sanity test.

More tests for specific auth error conditions will be added in a
later CL.

BUG=311411
TEST=sync_integration_tests:SyncAuthTest.Sanity
R=pavely@chromium.org, tim@chromium.org

Review URL: https://codereview.chromium.org/55293004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233648 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233648