How To Measure Usage and Notify Developers

1. Add your feature to the UseCounter::Feature enum.
2. Add a clever deprecation message to the big switch in UseCounter::deprecationMessage.
3. Instrument your code by either:
• Adding DeprecateAs=[your enum value here] to the feature's IDL definition (see window.performance.webkitGetEntries, for example).
• Adding a call to UseCounter::countDeprecation somewhere relevant (as we're dong for the prefixed Content Security Policy headers).