Add a script to audit RuntimeEnabledFeatures.in

Also shows history of when features were added/removed.

This could also be taught to show what features have been
abandoned or have not changed in several months, but that's
a v2 feature for now.

BUG=371746

Review URL: https://codereview.chromium.org/275213002

git-svn-id: svn://svn.chromium.org/blink/trunk@173805 bbb929c8-8fbe-4397-9dbb-9b2b20218538