One-time reset of UMA client ids and low entropy source.

One-time reset of UMA client ids and low entropy source.

As we've observed a "cloned install" effect in the wild,
doing this will de-clone such installs. (Which will work
in conjunction with other measures to prevent new instances
of cloned installs appearing).

Renames client id and low entropy source prefs so that they
are re-generated the next time Chrome runs. Makes the code
clear the old prefs, so that they are cleaned up in
existing local state files (in a future milestone, we can
remove them completely - http://crbug.com/357704).

(Note: Does not rename kMetricsPermutedEntropyCache as
that pref is reset when the low entropy id is re-generated.)

BUG=357618
TEST=Running an UMA-enabled build should result in local state
getting new values for user_experience_metrics.low_entropy_source2
and user_experience_metrics.client_id2 and removal of prefs
low_entropy_source and client_id.
NOTRY=true

Review URL: https://codereview.chromium.org/217083002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@260349 0039d316-1c4b-4281-b951-d872f2087c98