Remove unused StyleChangeReason::Plugin.

The "reason" was used when setNeedsStyleRecalc was used to trigger a
re-attachment of plugin renderers.

After landing https://codereview.chromium.org/596773002 a lazy re-attach is
done instead and this constant is no longer in use.

Review URL: https://codereview.chromium.org/615663003

git-svn-id: svn://svn.chromium.org/blink/trunk@182879 bbb929c8-8fbe-4397-9dbb-9b2b20218538