Remove old, unused per-plugin data from user prefs

Remove old, unused per-plugin data from user prefs

There is no longer a way for users to set per-plugin prefs (they can still be
set by other means, such as extensions). Clear this data from user prefs.

BUG=615738

[raymes, 2016-09-27 01:27:16]

raymes@chromium.org changed reviewers:
+ bauerb@chromium.org, msramek@chromium.org

[Bernhard Bauer, 2016-09-27 09:34:44]

lgtm

https://codereview.chromium.org/2376453003/diff/20001/components/content_sett...
File components/content_settings/core/browser/content_settings_pref.cc (right):

https://codereview.chromium.org/2376453003/diff/20001/components/content_sett...
components/content_settings/core/browser/content_settings_pref.cc:259: //
Careful: The returned value could be nullptr if the pref has never been
Tiny nit: I would use just "null" if it's more about the concept (so it reads a
bit more fluid, IMO).

https://codereview.chromium.org/2376453003/diff/20001/components/content_sett...
components/content_settings/core/browser/content_settings_pref.cc:338: for
(std::string pattern_str : remove_items) {
const std::string& so you don't copy them?

[msramek, 2016-09-27 11:56:15]

Sorry, not LGTM just yet, unless I'm missing something.

I checked out the tip of the tree and the chrome://plugins page has not been
removed yet. If I check a checkbox there, it will add a ["*", "*", <plugin id>]
exception.

https://cs.chromium.org/chromium/src/chrome/browser/ui/webui/plugins/plugins_...

This CL which translate that exception to ["*", "*", ""] which is the default
setting. In the debug version it will DCHECK. In the release version, it will
probably end up allowing all plugins (trumping whatever is in the default
provider).

While I'm very much in favor of removing this complexity, this CL should only
land after chrome://plugins is actually removed. There is nothing on that page
informing users that it's going to be deprecated and they shouldn't use it, so
someone's going to run into it.

[raymes, 2016-09-27 23:45:10]

> I checked out the tip of the tree and the chrome://plugins page has not been
> removed yet. If I check a checkbox there, it will add a ["*", "*", <plugin
id>]
> exception.

Oh man - I thought that was the case, and then I send guessed myself because of
the comment on https://codereview.chromium.org/2357663003/ and when I read the
code I thought that it used a different pref to set per-plugin settings from
about:plugins. Sorry! We should land https://codereview.chromium.org/2357663003/
for now.

[msramek, 2017-01-12 23:54:36]

As per https://codereview.chromium.org/2630443002/, I guess I can unblock this?
LGTM.

[msramek, 2017-01-12 23:56:07]

https://codereview.chromium.org/2376453003/diff/20001/components/content_sett...
File components/content_settings/core/browser/content_settings_pref.cc (right):

https://codereview.chromium.org/2376453003/diff/20001/components/content_sett...
components/content_settings/core/browser/content_settings_pref.cc:300: //
TODO(raymes): delete this code ~M57.
A lot of time has passed, so please update this :)