sync: reland sessions v2 on-by-default

This is a revert of: Revert of sync: turn on session sync v2 (https://codereview.chromium.org/130403003/)

This patch is equivalent to before, but we fixed some bugs in the meantime.

TBR=jyasskin@chromium.org,msw@chromium.org,zea@chromium.org
BUG=98892

Review URL: https://codereview.chromium.org/146343003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247153 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247153