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Changing the UsersPerSession UMA metric to count relative user amount changes for sessions which can do multi profile

The old absolute UMA counter was stored upon shutdown. Unfortunately only one in 33 users is shutting down the system properly. As such only a fraction of sessions were accounted for => the values were useless. Furthermore we have decided to only count sessions which could be multi profile sessions.

The new values are recording state changes like e.g. going from one user to two in a single session. As such you might get a resulting stat counter list of:

1: 320 => 320 - 15 - 4 - 1 = 300 single user sessions
2: 15  =>       15 - 4 - 1 = 10  two user sessions
3: 4   =>            4 - 1 = 3   three user sessions
4: 1   =>                    1   four user session

Note that it is not possible for a single user to leave a multi profile session - so the counter will always count up.

BUG=349055
TEST=none

Review URL: https://codereview.chromium.org/180243025

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255865 0039d316-1c4b-4281-b951-d872f2087c98