Updated comment in UseCounter's CSS mapping to mention new script to update Chrome's UMA strings.
Updated comment in UseCounter's CSS mapping to mention new script to
update Chrome's UMA strings.

Review URL: https://codereview.chromium.org/303483002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@174793 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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