How To Measure Usage and Notify Developers

1. Add your feature to web_feature.mojom's WebFeature.
2. Add a clever deprecation message to the big switch in UseCounter::deprecationMessage.
3. Instrument your code by:
• Adding DeprecateAs=[your enum value here] to the feature's IDL definition (see these examples).
• Adding a call to Deprecation::CountDeprecation somewhere relevant (as we're dong for the UserMediaRequest).