Add the PrefixedMediaAddKey value back to the UseCounter enum.
Add the PrefixedMediaAddKey value back to the UseCounter enum.

It was removed in 150962, which changed the position of the following items
in the enum. That's a bad thing. This CL simply adds it back to restore the
expected positions of the enum's values.

BUG=224791
TBR=ddorwin@chromium.org,tkent@chromium.org

Review URL: https://codereview.chromium.org/15735019

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@150969 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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