sync: couple histogram fixes

* Add Sync.Startup* histograms.
* Add recent ModelTypes to SyncModelTypes histogram enum (#s 25-31).

NOTRY=true
BUG=none

Review URL: https://codereview.chromium.org/180783009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254263 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254263