Uses GetDefaultProfileOrOffTheRecord() for ChromeOS guest mode.

In ChromeOS guest mode, notifier_groups_ becomes empty and it
causes a crash.
- it doesn't create a dummy profile at the end of RebuildNotifierGroups(),
  because this can be called too early and GetDefaultProfileOrOffTheRecord()
  may create a new off-the-record profile which causes Rebuild again.
- sync service is not available for the off-the-record

BUG=309854
R=dewittj@chromium.org
TEST=manually

Review URL: https://codereview.chromium.org/25629007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230691 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230691