Don't use plugin-specific content settings that are from user prefs

Don't use plugin-specific content settings that are from user prefs

Users used to be able to set plugin-specific settings in about:plugins but this
UI is going away so we shouldn't consider plugin-specific settings when resolving
the plugins content setting. We should eventually migrate/remove plugin-specific
settings since there is now no longer multiple plugins.

BUG=615738

[raymes, 2016-09-21 07:16:24]

raymes@chromium.org changed reviewers:
+ tommycli@chromium.org

[tommycli, 2016-09-21 16:52:35]

tommycli@chromium.org changed reviewers:
+ bauerb@chromium.org

[tommycli, 2016-09-21 16:52:35]

this is lgtm, but i think you will need bauerb also, since i don't own that
directory

[Bernhard Bauer, 2016-09-22 16:04:00]

Instead of ignoring these settings, shouldn't we remove them from preferences
instead? That would allow us to eventually remove the migration code – otherwise
we'd need to keep it around forever, no?

(Technically, removing them from user prefs would mean we'd sync them back to
older clients, but I don't really care, because the UI for this has been gone
for _ages_.)

[raymes, 2016-09-26 03:17:11]

On 2016/09/22 16:04:00, Bernhard (slow until Sep 27) wrote:
> Instead of ignoring these settings, shouldn't we remove them from preferences
> instead? That would allow us to eventually remove the migration code –
otherwise
> we'd need to keep it around forever, no?
> 
> (Technically, removing them from user prefs would mean we'd sync them back to
> older clients, but I don't really care, because the UI for this has been gone
> for _ages_.)

Oh - yeah I think you're right. I thought this was used by the current
about:plugins but it's not. I'll change this to remove the settings.

[raymes, 2016-09-27 23:45:58]

bauerb: as per https://codereview.chromium.org/2376453003/ can we land this?
Thanks!

[Bernhard Bauer, 2016-09-28 08:40:01]

On 2016/09/27 23:45:58, raymes wrote:
> bauerb: as per https://codereview.chromium.org/2376453003/ can we land this?
> Thanks!

OK, LGTM.

[raymes, 2016-09-28 23:15:57]

The CQ bit was checked by raymes@chromium.org

[commit-bot: I haz the power, 2016-09-28 23:16:17]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[raymes, 2016-09-28 23:16:59]

The CQ bit was unchecked by raymes@chromium.org

[raymes, 2016-09-28 23:41:15]

I realised we don't actually want to do this - we should respect the user's
about:plugins setting until we remove the page. I'll land the migration code
after we remove about:plugins