Get rid of the ability to unregister preferences.

Get rid of the ability to unregister preferences.

This was being used only in a couple of places that were clearing or
migrating old prefs, and as far as I understand, it is not necessary,
especially if we hide the names of those old prefs from users outside
of the class that clears them.

Also, do registrations of the prefs being migrated at up-front
registration time rather than in the migration functions.

TBR=ivankr@chromium.org
BUG=155525
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=183816

[Jói, 2013-02-20 15:34:49]

https://codereview.chromium.org/12330008/diff/1/chrome/browser/ui/prefs/prefs...
File chrome/browser/ui/prefs/prefs_tab_helper.cc (right):

https://codereview.chromium.org/12330008/diff/1/chrome/browser/ui/prefs/prefs...
chrome/browser/ui/prefs/prefs_tab_helper.cc:589:
prefs->ClearPref(kPrefNamesToMigrate[i].from);
Note, I moved this here since there seems no need to look up the preference
again to clear it, unless it didn't have its default value.

[Mattias Nissler (ping if slow), 2013-02-20 17:17:27]

LGTM with one request (feel free to ignore if you can't find the required
information from repository history within 15 min).

https://codereview.chromium.org/12330008/diff/5002/chrome/browser/prefs/brows...
File chrome/browser/prefs/browser_prefs.cc (right):

https://codereview.chromium.org/12330008/diff/5002/chrome/browser/prefs/brows...
chrome/browser/prefs/browser_prefs.cc:159: // Name of an old pref that is
cleared in MigrateUserPrefs.
I think it'd be nicer to actually have this in pref_names.h for consistency
(i.e. restore the old pref definition that I hope was there originally for
whoever used this). Might require some digging in history, but will probably
reduce the WTF-factor for readers.

[Jói, 2013-02-21 12:05:10]

https://codereview.chromium.org/12330008/diff/5002/chrome/browser/prefs/brows...
File chrome/browser/prefs/browser_prefs.cc (right):

https://codereview.chromium.org/12330008/diff/5002/chrome/browser/prefs/brows...
chrome/browser/prefs/browser_prefs.cc:159: // Name of an old pref that is
cleared in MigrateUserPrefs.
On 2013/02/20 17:17:27, Mattias Nissler wrote:
> I think it'd be nicer to actually have this in pref_names.h for consistency
> (i.e. restore the old pref definition that I hope was there originally for
> whoever used this). Might require some digging in history, but will probably
> reduce the WTF-factor for readers.

This wasn't in pref_names, it was in a .cc file that has been removed. I added a
comment with further explanation of what this pref name is, and referencing the
removed file so people can dig through history if they like.

[Mattias Nissler (ping if slow), 2013-02-21 13:10:21]

LGTM. Thanks for adding the comment explaining the backup pref.

[Jói, 2013-02-21 15:10:11]

TBR=ivankr@chromium.org for usage update for Prefs API change, in
chrome/browser/chromeos/login.

Cheers,
Jói

[Jói, 2013-02-21 15:10:52]

Committed patchset #4 manually as r183816 (presubmit successful).

[Ivan Korotkov, 2013-02-21 17:33:52]

LGTM