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: svn://svn.chromium.org/blink/trunk@173480 bbb929c8-8fbe-4397-9dbb-9b2b20218538