[Privet] Do not show notifications from guest and default profiles in ChromeOS

In ChromeOS, do not show notifications for guest and default profiles. When
starting the privet notification service, check if the profile has an
authenticated username. If it does, it is a normal user account; otherwise it
isn't.

BUG=349098

Review URL: https://codereview.chromium.org/186983003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255432 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255432
Remove the flash specific disk cache backend.

This disk cache backend is no longer under development.

R=davidben@chromium.org,agayev@gmail.com
TEST=None

Review URL: https://codereview.chromium.org/182093002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255433 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255433