Update pepper related per-file owner in content/common/OWNERS.

Make pluing* owners to be pepper* owners as well.

BUG=none
TEST=none
R=creis@chromium.org, jam@chromium.org

Review URL: https://codereview.chromium.org/33213002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229864 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229864