RFC proposal on Custom Handlers testing automation (#121)

Author: javifernandez

rfcs/testdriver_set_rph_registration_mode.md

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+# RFC X: Add Custom Handlers support to testdriver.js
+
+## Summary
+
+Add testdriver.js support for [Custom Handlers](https://html.spec.whatwg.org/multipage/system-state.html#custom-handlers/), by
+supporting a `setRPHRegistrationMode` command ([spec
+PR])(https://github.com/whatwg/html/pull/8267).
+
+## Details
+
+The proposed `setRPHRegistrationMode` command places the user agent in a mode
+where it will automatically accept or reject (depending on the mode) future
+Custom Protocol Handlers registrations. This allows full end-to-end
+testing of Custom Handlers API (eg, registerProtocolHandler), which normally [requires user
+interaction](https://html.spec.whatwg.org/multipage/system-state.html#dom-navigator-registerprotocolhandler)
+to confirm the user's consent to the protocol registration.
+
+Links:
+  - [WIP spec PR](https://github.com/whatwg/html/pull/8267)
+  - [WIP web-platform-tests implementation PR](https://github.com/web-platform-tests/wpt/pull/35792)
+  - [WIP Chromedriver implementation](https://chromium-review.googlesource.com/c/chromium/src/+/3865270)
+
+### Alternatives Considered
+
+I've also considered the usag of the already defined [Set Permission](https://w3c.github.io/permissions/#set-permission-command)
+extension command, implemented at least by Chromedriver. In order to use such command, we would need to define a
+new permission descriptor for the Custom Handlers feature. Such [proposal](https://github.com/whatwg/html/issues/7920)
+has been discussed and rejected by Chrome and Firefox implementors.
+
+## Risks
+
+This increases the API surface of testdriver.js, so it's more code to maintain
+or reason about.