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Two consumers:
- PluginStatusPrefSetter - was unused, so it's deleted.
- ChromePluginServiceFilter - the action taken when receiving the
  notification can be moved to the notification's source, i.e.
  PluginPrefs.

Note that the enable status can only change for the pdf viewer plugin,
so it wasn't a very aptly-named notification (at this point in history).

Bug: 268984
Change-Id: I1aebd7d90f8e70c3031d7e7911bb1388c8e88c8a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1671849Reviewed-by: Tommy Li <tommycli@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Auto-Submit: Evan Stade <estade@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#671914}