Merge 255865 "Changing the UsersPerSession UMA metric to count r..."

Merge 255865 "Changing the UsersPerSession UMA metric to count r..."

> 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

TBR=skuhne@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=258630

[Mr4D (OOO till 08-26), 2014-03-21 18:35:34]

Committed patchset #3 manually as r258630 (tree was closed).