Sync: Refactor the ProfileSyncService and sync setup flow to remove use of WebUI from PSS.

Also in this CL:
* Only call didShowPage on pages/overlays if they're not already visible.
* Remove buggy and unused logic in shouldClosePage.

BUG=85935
TEST=none

R=csilv@chromium.org,binji@chromium.org

Review URL: http://codereview.chromium.org/7093004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@88941 0039d316-1c4b-4281-b951-d872f2087c98