sync: a few more fixes for sessions V2

Caught by integraton tests. A nice part of this change is that in the v2 world, we no longer need "QuitLoopForSubtleTesting" and "test_weak_factory" stuff in the production code; it lives exclusively in SessionsHelper.

BUG=98892
R=zea@chromium.org

Review URL: https://codereview.chromium.org/137163002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244621 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244621