Added syncable priority pref for ChromeOS first-run UI.

First-run UI will be shown for user only once.
Also changed place where we decide to show or not to show first-run tutorial
for user. Now decision made in the same place, where we made decision for
"Tips & Tricks" app.

BUG=269739,323261

Review URL: https://codereview.chromium.org/116543002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241315 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241315