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This changes adds four histograms to UMA. Profile.NumberofSwitches
collects the number of profile switches happening over a session.

The others are:

- Profile.ProfileSwitch.ToOpenedProfile counts the number of switches to
  opened profiles and buckets depending on the UI flow used to switch.
- Profile.ProfileSwitch.ToUnopenedProfile does the same but for switches
  to unopened profiles
- Profile.ProfileSwitch aggregates the data from the previous two

BUG=448982

Review URL: https://codereview.chromium.org/844193005

Cr-Commit-Position: refs/heads/master@{#313951}