Added comment to mention new script to update the UMA mapping.
Added comment to mention new script to update the UMA mapping.

The script produces patches such as the one at
https://codereview.chromium.org/272503003/

Review URL: https://codereview.chromium.org/264403002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@173480 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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